I believe that there are two different "kits" to upgrade an M2 to an M2A1. There is an FN kit and a General Dynamics kit. They are different designs and components are not interchangeable between the two. However, I believe M2s fitted with either kit are both known as M2A1s with no differentiation between the two, unless someone knows differently.
U.S. Ordnance also has produces an M2A2 that is different from either of the previously mentioned models, although I do not believe that U.S. forces have purchased any.