Trying to figure out the secret sauce for the Simpson. It appears that the only magical component aside from a few precision resistors and some LT1001 op amps is a small Dale transformer which I gather would be unobtainium if it failed. I still haven't tracked down the signals but it looks like that, after passing through the DUT, the 40hz sq wave goes into the transformer and the secondary is then rectified and, I assume, measured by the Maxim 4.5 digit ADC/LCD driver. I guess the trick is getting usable results from a 100uV applied signal.
Still no luck getting a service manual or schematic.