That does not necessarily mean that they no longer have earlier marks of the system. It was policy in the Red Army to retain equipment, in the event of war as reserve groups were called up to replace losses in action the "reservists" were reunited with the equipment that they trained and used during the time of their conscription. This policy had certain advantages and certain drawbacks, the upper echelons of the Red Army obviously thought that the former outweighed the latter.

"Inside the Soviet Army (ISBN 0-241-10889-6; Hamish Hamilton, 1982; also published in the United States, Prentice Hall, ISBN 0-02-615500-1), by Viktor Suvorov, which describes the general organisation, doctrine, and strategy of the Soviet armed forces"

They haven't changed.