P4, but it shows that ARMS mounts work on them. I shot this one about 200rds for a review I did on these rail systems awhile back. It is a decent rail system, really light and solid, IF you mount it correctly. If the setscrews that hold it come loose...........................see pic below for the result @ 25yds. IMO, Loctite the setscrews that hold the unit on and Loctite the locking setscrews that go on top of the setscrews, to make sure they don't come loose. POF says that this is not req'd, but I would for added peace of mind.
Loose rear setscrew result at 25 yds: