The differences are:
120 : has the ISO dial on the top of the back.
120-I : has the ISO dial on the back of the back, so its easier to see and adjust when the camera has a prism attached, it also has a was of adjusting the exposure +/- in 1/3 stops
120-B : doesn't have an ISO dial or connectors
Otherwise they are basically identical, and can be used on any sq series camera.
I've got an SQ-A and I use both 120 & 120-I backs.
Are you only shooting at 1/500 sec? If you don't have batteries in the camera, it will only fire at 1/500 sec. The shutter speeds are controlled electronically in all Bronica SQ series cameras.