Part 1 - Would the M2 comp belts work?
If you have a M4, then yes. The all black belts on the M2 and M4 are both part number 72-11-7-259-391. Since those are interchangeable, the M2 Comp seatbelts would work in the M4.
If you have the M3, or if you're trying to put these into a F31...then I think the answer is more complicated.
Sorry, picture heavy thread coming here. First, here's pictures of the rear left belt for the M2:
Overview
From the left side
From the back
From the right side
From the bottom
I would say, if the mounting point looks the same, then you're
probably good. However, wanted to point out something that relates to the function of the belt. The part that locks the belt when pulled quickly is a little mettle marble that sits in a chamber to allow movement of the belt when oriented upright and pulled slowly. Here's a view of what that looks like:
This is looking at the right hand side belt with the cover removed, the purple thing with the yellow foot and chrome ball is the thing I'm talking about. If you tilt the assembly, the ball doesn't rest all the way at the bottom and it locks the belt from moving. So assuming the assembly is exactly the same, I think it would have to mount in the same orientation for it to work.