OK .......the T-54M. Who used it ? Russia ? Or were they only exported? Your date is 1977 but I've seen others that say .....
# T-54M (mid-1960s) – Upgrade program to bring T-54s up to T-55 standard.
# T-54M (1977) – Additional upgrades, including OPVT snorkel and KTD-1 laser rangefinder.
which indicates this was done in stages and the first stage made the T54m essentially the same tank as a T55M and "Mid sixties" could mean some were started in 1963 and they were all upgraded by 1968 during overhauls......maybe.
Other sources (
http://www.globalsecurity.org/milita...ussia/t-54.htm ) say "T-54M: T-54 upgraded to the T-55M standard. See T-55M." so aside from one being called "T-54" and one being called "T-55" they are essentially the same tank and although I appreciate that this would seem to be "more historical" it's also redundant. If it genuinely fills an important spot in a nations OOB I would be happy to add it. If all we are doing is renaming a T55m to T54M there seems little point
This from Wikipedia. I don't really like quoting Wiki but here goes...."The T-54 and T-55 tanks are very similar and difficult to distinguish visually. Many T-54s were updated to T-55 standards. Soviet tanks were factory-overhauled every 7,000 km, and often given minor technology updates."
So every overhaul some new little gadget may have replaced an old one or a new thingy is tacked on which means any OOB is just an approximation of reality as it is impossible to say with 100% certainty that at any given date the OOB of any nation exactly mirrors reality.
Don