I did a test of the newer calibrator board signal stability. I fed the input with the 10 MHz sine wave from an HP-5071A cesium beam oscillator and fed the output to a TAPR TICC. I used Lady Heather to calculate the xDEVs of the output signals. At 10,000 seconds the ADEV value was in the 6E-15 range... not too shabby!
The outputs do have ringing on them, but the counter's calibration routine uses the zero crossing point which is quite stable.
The HP docs say for the gain calibration to input a 5.000V +/- 1mV signal. The LM4040A voltage reference chips that I am using seem to be within +/- 2 mV. Should not be an issue since the A/D and D/A chips in the counter are not that accurate. You can always use your own voltage source for the gain cal.