I use this on my PE 68/3000 it fits well. My friend uses it on his crossfire 68/4500 fits it perfectly fine. NEver had problems with it. It's purpose is to keep dirt and stuff out of the tank and it does a great job. Just put it on after the tank is screwed in.
It's a very cool nipple cover, get one. If you have an old psycho drop you might have a problem tho.