I’m not sure it stands for anything. For both models the base model has no letter after the number and quite simple sights, the A has better open sights, and the B has an aperture rear sight, all on a sporting stock. M as you say is Match with the appropriate stock and the aperture rear sight and complex front sight.
The 50E, by the way, was a U.K.-specific bargain model with simple rear sight and no rail on the cylinder. E probably stands for England, if it stands for anything.