SOOooo ...

I retrieved my SMC 28mm and, fiddling with the two variable resistors at EV16 and EV8 I've gotten the open aperture metering circuit well inside what I'd regard as acceptable readings compared to my sekonic and checked against my F100 with the same lens fitted via an adapter. Not too much adjustment needed either, which suggests that the two CDS cells (or whatever is in the spotmatic) are ok.
The difficulty is with the stop down metering. Even adjusting the seperate variable resistor for the stop down metering to its extreme I cannot get the stop down metering to read at anything other than about 2 stops underexposed. This has me confused because it's the simpler of the two circuits, and it's reading off the same two CDS cells which appear to be just fine. Anyone suggest any possible avenues of investigation ?

Interesting (by which I mean annoying) side note, you have to put the top back on the spottie to actually get the meter to read correctly. That led to some perplexed fiddling and weird erroneous readings until I worked it out