HERE YA GO...
The Benelli M2 is a warmed over bastardized verson for the M1. The M2 barrel will fit the M1 receiver, (and vice versa). The M2 forearm will fit the M1 barrel,
*but the M1 forearm will need some slight modifications to fit the M2 barrel, (easy to do).*
The M2 receiver has been recontoured mostly at the rear where the butt stock meets it, so the shape of the M2 stock will not match the M1 without corners of the receiver exposed. The M1 stock can be shaped to fit the M2 receiver