Check the Lycoming Type Certificate Data Sheet
There are multiple differences between the B1B and the M1B engine configurations. Trace it back from the TCDS:
The IO-360-M1B is similar to IO-360-M1A except it has a rear mounted propeller governor and impulse magneto; The M1A is similar to IO-360-B1E except it has a front inlet fuel injector, prop governor on front of crankcase and retard magneto; The B1E is similar to the IO-360-B1B except for the oil sump, induction manifolding and alternate magnetos; The B1B is similar to IO-360-B1A except it has a Bendix RSA-5AD1 injector and fuel pressure limit at inlet to diaphragm pump;
So, the B1B has a front-mounted prop governor, a different oil sump and induction manifolding, and Bendix mags with impulse coupling.
The B1B also weighs a little less than the M1B (11 pounds lighter according to the TCDS)
All the deltas in configurations can be located on the TCDS sheet for the Lycoming IO-360 engine available on the FAA website.
Hope this helps.