I already discussed how, due to a mistake, the Light Tank M2A4 was "spotted" in the ranks of the Red Army. Since serial numbers were present in the listing, it was fairly easy to figure out that these were actually fairly common Light Tanks M3.
Let's take a look at a similar situation. Once in a while, you can see rumours of the Medium Tank M2 being supplied via Lend Lease. It's not hard to be confused when you see something like this:
In this document the 201st Tank Brigade is complaining about all of the equipment that it's lacking. There is a shortage of every kind of staff, no artillery, hardly any machineguns, only one APC, and no heavy or light tanks. Instead of 20 T-34s, they received 22 "M-2" tanks. Makes sense, one medium tank replaces another, case closed.
There is, however, one issue. Only 18 Medium Tanks M2 were ever built. Maybe it's possible that the M2A1 was lumped under the same index? Let's keep going through the brigade's documents.
Eventually, the brigade scrounges up another handful of "M-2" tanks and some T-60s and goes off to fight. The amount of "M-2s" is dutifully recorded, until, one day, someone else fills out the unit's combat journal for the day, and the mystery is finally solved.
The 21 remaining "M-2" tanks turn into... Mk.II tanks. As in Infantry Tank Mk.II, or the very ordinary Matilda. Skipping forward another month, we see "artillery Mk.II tanks" and "armour piercing Mk.II tanks" recorded in the brigade's journal, quite obviously a reference to Matilda CS tanks armed with howitzers, as opposed to the regular 2-pounder gun, which only fired AP shot.