Chris,
The presence of an A, B, etc on the end of a part number usually indicates that it's different lifecyle generations of the same product. The A and B units *should* be functionally equivalent but had different introduction and discontinuance dates. Please note that I/O module for this enclosure is no longer available for sale as a new part from HPE so I would expect you would need to source the modules, and/or shelf w/modules, via a used/refurbished route.
Thanks
Mike