Just found this summary...
"SQ-Ai:
You may add on a motor drive, the SQ-i motor.
There is a connection port for off-the-film plane flash control.
The body has a multi exposure warning in the viewfinder
A 16 second calibrated shutter speed
"B" (bulb) setting on the shutter speed dial.
The backs between the previous models and the SQAi are fully compatible
All accessories for the SQ, SQ-A, and SQ-Am were compatible with this body.
SQ-A:
Is an updated SQ.
Auto metering capability with the AE finder S.
Mirror lock-up lever was added.
All accessories for SQ cameras fit the SQ-A.
SQ-B:
Is almost the same as the SQ-A, no metered prism.
The shutter speed range is the same as the SQ-A (8 sec to 1/500).
The SQ-B has mirror lock-up.
SQ:
This is the one I have and is the precursor to the models above. No metering, no mirror lock-up, same shutter speed range as the SQ-A, SQ-B."
After digging around a bit, it would appear that the 'lack of electronics' is the fundamental basis for differences...so electronic contacts for the film back to communicate ISO are missing; contacts for metering prism power and interface to lens aperture control is missing; electronic link between lens and meter and back are missing. So if you want a purely mechanical camera (except the shutter speeds are still controlled from body to leaf shutters in the lenses), the SQB may be the way to go. OTOH, how much price difference is there, now that film cameras have plummeted in value on the open market?