In the history of the MTA, LIRR received the M1, M3, & M7 models while Metro North received the M1A, M3A & M7A models, which were the same cars with slight differences as requested/needed by each railroad.
With the M9, someone either didn't care to maintain that or didn't know. Had MNR decided to purchase M9 cars, they also would have been M9, not M9A (Which they should have been)
To further corrupt this scheme, the M9A will be a different car altogether than the M9, perhaps not even compatible, it should really be the M11
With the M9, someone either didn't care to maintain that or didn't know. Had MNR decided to purchase M9 cars, they also would have been M9, not M9A (Which they should have been)
To further corrupt this scheme, the M9A will be a different car altogether than the M9, perhaps not even compatible, it should really be the M11