T-54/55 versions

[brummbaer]

From Osprey book:
"The T-54M program begun in the mid-60s to upgrade the T-54 with new features and components from the T-55 program. To further confuse matters a second T-54M program was developed in 1977 by the Omsk KTMB with further upgrades, including the addition of the OPVT system and the KTD-1 LRF... The T-54AM upgrade in the mid 60s brought the T-54A closer to the T-55A standards, including the increase in stored ammonition and engine."

So I'd say you're right with the two stage modernization;
though it doesn't state that clearly, my guess is that T-54M/AM were kept by the Red Army at least in mobilization units and storage, and they kept selling them for years.

My point is trying to make some clearness about the models that were developed, thence your choice if include them or not: I agree that synthesis of redundant models is better.