it actually is chanbered for 3 1/2 and is reciever is identical to the other models. Remmington just say's don't use 3 1/2 because the long tube tends to crush 3 1/2 shells if loaded to capacity. Loading 3-4 supposedly is not an issue.
As I think through this, I think your statement makes sense that the Benelli is 1/2" shorter. I think that's true for the M2 but I have the Super Black Eagle which takes 3 1/2 so I think we are both right.
So I'm hoping bottom line is the SBE lifter is identical but the M2 probably is not.