BR2A - Fits D600, FE2 and FM3A in all three positions. The fit is tight, tightest on the F600 (this is my newest camera so the mount may be less worn)
BR2 - One slot does not click in any of my D600, FE2 or FM3A. Looking at the mount from the side it appears the lens locking slot is in slightly the wrong place so can't engage with the lens locking pin. Another slot fits the D600 and FE2 but not the FM3A - it seems to slide over the lens locking pin but the pin never engages. Maybe the slot is a little too short or narrow for the pin to go in. The third position works for all three cameras. Again, the D600 is tightest.
The only difference I can see between the BR2 and BR2A is that the lens locking slots are very slightly longer and wider, so they have less trouble engaging with the lens locking pin on the camera.