Since I've got known measured LTZ ref in my lab, I'm running DCV/DCV Ratio measurements last days using 3458A.
Here's setup and ratio on first module.

Reference unit (A9 PCB from HP3458A) is in DIY TEC thermostat, at +23.4°C +/-0.01°C.
Unit under test - my KX LTZ reference design is at ambient temp, battery powered.
Second module comparisonA9 Reference measurements, with various temperatures (constant +23.4, temp at which it was measured by calibrated last summer 3458A, ramp +23.4 to +46C in 0.1C/10 minute speed, constant +50C). Measured value is +7.184609VDC, so after a day of warmup calibrated my 3458A to CAL 7.184609VDC.
I could not figure out what is tempco of A9 Ref. It's stock module, no tweaks/mods. Originally I wanted to use K2002 in parallel to measure A9 ref value as well, but impedance of meter not high enough and cause ratio to change ~4ppm.