So I feel daft now, as it turns out mine is an M2 NOT an M2A1. I was speaking to a chap called Logan in the US and he pointed out that although mine has some "later" features (pedestal lights and twin idler springs), it is missing the cross member that was added under the forward end of the rear floor to support the pulpit. Now while there is a lot missing due to wear and tear, this piece has simply never been fitted!
After 75+ years then it seems this old girl will be getting an upgrade, sadly there's no way I'll be able to afford the skate and trollies to make her an M2!
Good look with your research. Bumper and bogies seem to be the places to look for extra numbers.