That reminds me I need to calibrate my 2400, after taking it apart to do measurements for you.
My K2400 also non-original, I have modified it with LM399 reference for each oomph.

Not that it matters really, but was fun easy mod.
Calibration
Python program and E5810A aid adjustment automation and verification.
Keithley Model 2400 - detected, S/N 0788393, Version: C32 Oct 4 2010 14:20:11/A02 /H/H
Pre-cal K2400 Calibration data
0.2 DCV Range, SENS:DATA? = -1.375303E+00,-1.721084E-05,-1.375407E+00,+0.000000E+00
0.2 DCV Range, SOUR:DATA? = +2.764157E+05,+9.152109E+02,-2.764087E+05,+9.467695E+02
2.0 DCV Range, SENS:DATA? = -1.375422E+00,-1.811981E-05,-1.375288E+00,+0.000000E+00
2.0 DCV Range, SOUR:DATA? = +2.761708E+04,+2.052578E+02,-2.761636E+04,+2.083945E+02
20.0 DCV Range, SENS:DATA? = -1.375513E+01,-1.375198E-03,-1.375385E+01,+0.000000E+00
20.0 DCV Range, SOUR:DATA? = +2.761523E+03,+2.085977E+02,-2.761447E+03,+2.050625E+02
200.0 DCV Range, SENS:DATA? = -1.375421E+02,-1.382446E-02,-1.375290E+02,+0.000000E+00
200.0 DCV Range, SOUR:DATA? = +2.761715E+02,+2.086133E+02,-2.761635E+02,+2.050547E+02
1.0e-06 DCI Range, SENS:DATA? = -6.845910E-07,-9.356427E-11,-6.845222E-07,+0.000000E+00
1.0e-06 DCI Range, SOUR:DATA? = +5.548882E+10,+2.040664E+02,-5.548042E+10,+2.104687E+02
1.0e-05 DCI Range, SENS:DATA? = -6.890184E-06,-5.320544E-10,-6.889600E-06,+0.000000E+00
1.0e-05 DCI Range, SOUR:DATA? = +5.513167E+09,+2.019297E+02,-5.512313E+09,+2.127148E+02
1.0e-04 DCI Range, SENS:DATA? = -6.869998E-05,-4.991307E-09,-6.869353E-05,+0.000000E+00
1.0e-04 DCI Range, SOUR:DATA? = +5.529379E+08,+2.017656E+02,-5.528538E+08,+2.128203E+02
1.0e-03 DCI Range, SENS:DATA? = -6.857473E-04,-4.994217E-08,-6.856770E-04,+0.000000E+00
1.0e-03 DCI Range, SOUR:DATA? = +5.539511E+07,+2.017305E+02,-5.538684E+07,+2.128320E+02
1.0e-02 DCI Range, SENS:DATA? = -6.872036E-03,-5.178154E-07,-6.871379E-03,+0.000000E+00
1.0e-02 DCI Range, SOUR:DATA? = +5.527761E+06,+2.018398E+02,-5.526910E+06,+2.127383E+02
1.0e-01 DCI Range, SENS:DATA? = -6.876468E-02,-5.178154E-06,-6.875831E-02,+0.000000E+00
1.0e-01 DCI Range, SOUR:DATA? = +5.524166E+05,+2.018516E+02,-5.523274E+05,+2.127734E+02
1.0e+00 DCI Range, SENS:DATA? = -6.887156E+00,-5.303621E-04,-6.886814E+00,+0.000000E+00
1.0e+00 DCI Range, SOUR:DATA? = +5.518421E+04,+8.602578E+02,-5.517694E+04,+9.663477E+02
HP 3458A detected
TEMP? = 37.5 C
DCV Calibration step -0.2 mV , measured : -1.999948835E-01 VDC, deviation -25.583 ppm
DCV Calibration step 0.2 mV , measured : 1.999991793E-01 VDC, deviation -4.104 ppm
DCV Calibration step -2 V , measured : -1.999991983E+00 VDC, deviation -4.008 ppm
DCV Calibration step 2 V , measured : 1.999986310E+00 VDC, deviation -6.845 ppm
DCV Calibration step -20 V , measured : -1.999999159E+01 VDC, deviation -0.421 ppm
DCV Calibration step 20 V , measured : 1.999993647E+01 VDC, deviation -3.177 ppm
DCV Calibration step -200 V , measured : -1.999974777E+02 VDC, deviation -12.612 ppm
DCV Calibration step 200 V , measured : 1.999964677E+02 VDC, deviation -17.662 ppm
DCI Calibration step -1 uA , measured : -9.999818293E-07 ADC, deviation -18.171 ppm
DCI Calibration step 1 uA , measured : 9.999825290E-07 ADC, deviation -17.471 ppm
DCI Calibration step -10 uA , measured : -9.999831268E-06 ADC, deviation -16.873 ppm
DCI Calibration step 10 uA , measured : 9.999901602E-06 ADC, deviation -9.840 ppm
DCI Calibration step -100 uA , measured : -9.999936766E-05 ADC, deviation -6.323 ppm
DCI Calibration step 100 uA , measured : 9.999918902E-05 ADC, deviation -8.110 ppm
DCI Calibration step -1 mA , measured : -9.999886353E-04 ADC, deviation -11.365 ppm
DCI Calibration step 1 mA , measured : 9.999875093E-04 ADC, deviation -12.491 ppm
DCI Calibration step -10 mA , measured : -9.999853206E-03 ADC, deviation -14.679 ppm
DCI Calibration step 10 mA , measured : 9.999953624E-03 ADC, deviation -4.638 ppm
DCI Calibration step -100 mA , measured : -9.999813773E-02 ADC, deviation -18.623 ppm
DCI Calibration step 100 mA , measured : 9.999842788E-02 ADC, deviation -15.721 ppm
DCI Calibration step -1 A , measured : -9.999645533E-01 ADC, deviation -35.447 ppm
DCI Calibration step 1 A , measured : 9.999774736E-01 ADC, deviation -22.526 ppm
DCI Verification step Zero A , measured : -1.788568192E-11 ADC
DCI Verification step -1 nA , measured : -9.993935778E-10 ADC, deviation -606.422 ppm
DCI Verification step 1 nA , measured : 9.888950700E-10 ADC, deviation -11104.929 ppm
DCI Verification step -10 nA , measured : -1.000216363E-08 ADC, deviation 216.363 ppm
DCI Verification step 10 nA , measured : 9.988446699E-09 ADC, deviation -1155.330 ppm
DCI Verification step -100 nA , measured : -1.000165300E-07 ADC, deviation 165.300 ppm
DCI Verification step 100 nA , measured : 1.000043848E-07 ADC, deviation 43.848 ppm
DCI Verification step -1 uA , measured : -9.999055619E-07 ADC, deviation -94.438 ppm
DCI Verification step 1 uA , measured : 9.998880573E-07 ADC, deviation -111.943 ppm
DCI Verification step -10 uA , measured : -9.999861188E-06 ADC, deviation -13.881 ppm
DCI Verification step 10 uA , measured : 9.999910553E-06 ADC, deviation -8.945 ppm
DCI Verification step -100 uA , measured : -9.999887831E-05 ADC, deviation -11.217 ppm
DCI Verification step 100 uA , measured : 9.999839386E-05 ADC, deviation -16.061 ppm
DCI Verification step -1 mA , measured : -9.999803701E-04 ADC, deviation -19.630 ppm
DCI Verification step 1 mA , measured : 9.999745446E-04 ADC, deviation -25.455 ppm
DCI Verification step -4 mA , measured : -4.000350359E-03 ADC, deviation 87.590 ppm
DCI Verification step 4 mA , measured : 4.000267999E-03 ADC, deviation 67.000 ppm
DCI Verification step -10 mA , measured : -9.999839193E-03 ADC, deviation -16.081 ppm
DCI Verification step 10 mA , measured : 9.999906841E-03 ADC, deviation -9.316 ppm
DCI Verification step -20 mA , measured : -2.000352007E-02 ADC, deviation 176.003 ppm
DCI Verification step 20 mA , measured : 2.000418054E-02 ADC, deviation 209.027 ppm
DCI Verification step -100 mA , measured : -1.000000364E-01 ADC, deviation 0.364 ppm
DCI Verification step 100 mA , measured : 9.999876289E-02 ADC, deviation -12.371 ppm
DCI Verification step -500 mA , measured : -5.001218448E-01 ADC, deviation 243.690 ppm
DCI Verification step 500 mA , measured : 5.001075087E-01 ADC, deviation 215.017 ppm
DCI Verification step -1 A , measured : 1.000053606E+00 ADC, deviation 53.606 ppm
DCI Verification step 1 A , measured : -1.000009712E+00 ADC, deviation 9.712 ppm
DCI Verification step Zero A , measured : 1.384233768E-08 ADC
DCV Verification step 00 (0.000000000E+00), measured : -8.871845474E-06 VDC
DCV Verification step 01 (-2.000000000E-03), measured : -2.010135899E-03 VDC, deviation 5067.950 ppm
DCV Verification step 02 (2.000000000E-03), measured : 1.990735405E-03 VDC, deviation -4632.297 ppm
DCV Verification step 03 (-2.000000000E-02), measured : -2.001099592E-02 VDC, deviation 549.796 ppm
DCV Verification step 04 (2.000000000E-02), measured : 1.999615615E-02 VDC, deviation -192.193 ppm
DCV Verification step 05 (-2.000000000E-01), measured : -2.000073866E-01 VDC, deviation 36.933 ppm
DCV Verification step 06 (2.000000000E-01), measured : 1.999953503E-01 VDC, deviation -23.248 ppm
DCV Verification step 07 (-1.000000000E+00), measured : -1.000055863E+00 VDC, deviation 55.863 ppm
DCV Verification step 08 (1.000000000E+00), measured : 1.000063591E+00 VDC, deviation 63.591 ppm
DCV Verification step 09 (-2.000000000E+00), measured : -2.000200776E+00 VDC, deviation 100.388 ppm
DCV Verification step 10 (2.000000000E+00), measured : 2.000241744E+00 VDC, deviation 120.872 ppm
DCV Verification step 11 (-1.000000000E+01), measured : -1.000062483E+01 VDC, deviation 62.483 ppm
DCV Verification step 12 (1.000000000E+01), measured : 1.000049089E+01 VDC, deviation 49.089 ppm
DCV Verification step 13 (-1.500000000E+01), measured : -1.500109513E+01 VDC, deviation 73.009 ppm
DCV Verification step 14 (1.500000000E+01), measured : 1.500119825E+01 VDC, deviation 79.883 ppm
DCV Verification step 15 (-2.000000000E+01), measured : -2.000004998E+01 VDC, deviation 2.499 ppm
DCV Verification step 16 (2.000000000E+01), measured : 2.000003855E+01 VDC, deviation 1.928 ppm
DCV Verification step 17 (-1.000000000E+02), measured : -1.000070850E+02 VDC, deviation 70.850 ppm
DCV Verification step 18 (1.000000000E+02), measured : 1.000052509E+02 VDC, deviation 52.509 ppm
DCV Verification step 19 (-2.000000000E+02), measured : -1.999974277E+02 VDC, deviation -12.861 ppm
DCV Verification step 20 (2.000000000E+02), measured : 1.999966999E+02 VDC, deviation -16.500 ppm
DCV Verification step 21 (0.000000000E+00), measured : -2.192978188E-04 VDC
Post-cal K2400 Calibration data
0.2 DCV Range, SENS:DATA? = -1.375304E+00,-1.619756E-05,-1.375398E+00,+0.000000E+00
0.2 DCV Range, SOUR:DATA? = +2.764154E+05,+9.151484E+02,-2.764104E+05,+9.473320E+02
2.0 DCV Range, SENS:DATA? = -1.375421E+00,-1.704693E-05,-1.375296E+00,+0.000000E+00
2.0 DCV Range, SOUR:DATA? = +2.761707E+04,+2.052461E+02,-2.761638E+04,+2.084570E+02
20.0 DCV Range, SENS:DATA? = -1.375512E+01,-1.420975E-03,-1.375377E+01,+0.000000E+00
20.0 DCV Range, SOUR:DATA? = +2.761523E+03,+2.086992E+02,-2.761452E+03,+2.049922E+02
200.0 DCV Range, SENS:DATA? = -1.375419E+02,-1.417542E-02,-1.375297E+02,+0.000000E+00
200.0 DCV Range, SOUR:DATA? = +2.761712E+02,+2.087305E+02,-2.761636E+02,+2.049766E+02
1.0e-06 DCI Range, SENS:DATA? = -6.845868E-07,-9.220003E-11,-6.845208E-07,+0.000000E+00
1.0e-06 DCI Range, SOUR:DATA? = +5.548899E+10,+2.039805E+02,-5.548046E+10,+2.105508E+02
1.0e-05 DCI Range, SENS:DATA? = -6.890186E-06,-5.265974E-10,-6.889579E-06,+0.000000E+00
1.0e-05 DCI Range, SOUR:DATA? = +5.513168E+09,+2.018594E+02,-5.512315E+09,+2.127813E+02
1.0e-04 DCI Range, SENS:DATA? = -6.870023E-05,-4.889444E-09,-6.869372E-05,+0.000000E+00
1.0e-04 DCI Range, SOUR:DATA? = +5.529382E+08,+2.016289E+02,-5.528539E+08,+2.129570E+02
1.0e-03 DCI Range, SENS:DATA? = -6.857464E-04,-4.877802E-08,-6.856792E-04,+0.000000E+00
1.0e-03 DCI Range, SOUR:DATA? = +5.539507E+07,+2.016250E+02,-5.538670E+07,+2.129336E+02
1.0e-02 DCI Range, SENS:DATA? = -6.872036E-03,-5.168840E-07,-6.871358E-03,+0.000000E+00
1.0e-02 DCI Range, SOUR:DATA? = +5.527748E+06,+2.017773E+02,-5.526900E+06,+2.128164E+02
1.0e-01 DCI Range, SENS:DATA? = -6.876391E-02,-5.215406E-06,-6.875829E-02,+0.000000E+00
1.0e-01 DCI Range, SOUR:DATA? = +5.524202E+05,+2.018477E+02,-5.523324E+05,+2.127852E+02
1.0e+00 DCI Range, SENS:DATA? = -6.887750E+00,-5.336404E-04,-6.886620E+00,+0.000000E+00
1.0e+00 DCI Range, SOUR:DATA? = +5.518539E+04,+8.598516E+02,-5.517846E+04,+9.664961E+02
Calibration complete!