In their BOM and schematics, Keithley names general purpose diodes as CR1, CR2,.., whereas diodes with higher requirements were named VR1, VR2,..
In the Keithley 199, they use an 1N4579 as the reference, it's a 6.4V / 5ppm/K compensated zener type, see schematic attached, it's VR2.
They specify for this 5 1/2digit DMM: 30ppm for 24h 23 +/-1°C and 60ppm for 90d / 23+/-5°C, which fits well into the zener spec.
Therefore, also in the 2400 SMU they may use a compensated zener reference, presumably VR600 in conjunction with VR 602, 603.. but I 'm missing resistors nearby in that part of the circuit.. And there is no other leaded glass diode around the A/D, therefore VR600 for sure is something like an 1N4579.
Maybe, if Dave would measure TP232 (Vref), and then the voltage across these VR diodes, maybe this would match.
In the end, the stability / accuracy specifications of the 2400 SMU are nearly an order of magnitude worse in DMM mode, than the DMM 199, for example.. Therefore, the usage of a simple zener reference is very probable.
Frank
PS: I'm so blind, today! It's all in the Keithley SMU 2400 manual!
page 6-9: replaceable parts: VR600 = 1N4579, ZD 6.4V
page 4-11: analog circuitry check: TP232 = 6.4V +/-6V (? ? ?)
So it's exactly, as assumed, and Keithley is very conservative with their components, over the years.