I have a newer ACOG that now has the mounting bosses for the RMR mount RM35 built onto the eyepiece. With this in mind, there is minimal room to mount this rubber eyepiece. It still mounts ok without it falling off. What I may do is trim pieces off the side to clear the bosses so that it will be a little more secure.
Best feature of this rubber piece is that it aids me in quick positioning to look through the ACOG since it's positioned well for a reference of eye relief.
Pros: helps you locate eye relief quickly
This review was written in the old system and had content requirements that are different than reviews written today.