A study out a few years ago said that 80% of the heavy tanks defending Moscow in winter 41/42 were Brit or American. I don’t know what that means if we didn’t’ have any heavy tanks unless they were referring to the Grant/Lee.
The M3 (Lee/Grant) was a medium tank. At the time we may have shipped some to the Reds. The Brits provided the bulk of lend lease tanks in late 41. These could have been M3s or their Matildas. I think at the time only the Soviets built “Heavy” tanks. This would have been the KV-1 series in late 41.