This is the longer (Version 2) of the Buffer Rear Stop. It is 3.656" long compared to the original and shorter 3.063"' version 1.
Originally, the shorter buffer rear stop was used with a TA50131 Buffer Guide Spacer. Those 2 parts (the spacer and the short rear stop) are now deprecated and replaced by this single piece, the longer Buffer Rear Stop v2.
We no longer have the shorter Rear Stop so if that's what you're looking for you'll just need to get this longer one and then toss out your Buffer Guide Spacer when you put in this new longer version.