M1A2 drive sprocket question

thanks, I thought it might be something like that.

You could use either, really. The one with the scalloping on the "inside" of the ring is the later, lighter   version.  The one without is the earlier version as originally fitted to M1A1's. They are pretty much interchangeable.

If you are doing a particular vehicle at a particular time and place, you may need to refer to photos of the specific subject.