The RPM is their Elite series of mounts and offers micro-adjustment capabilities which the RPA series does not. The RPA series dates back as far as I can remember, which means that the design is 20+ years old. The RPM series is newer and is a FAR better mount than the RPA series.
It should be noted that I would NEVER buy a custom plate (SLB), but would get the RPMAU which is a universal bracket. The universal bracket is absolutely top of the class in terms of quality and is what I am using on my JVC projector without any problems.
These pop up on eBay from time to time and can be had for under $100 quite often when they do pop up. If you are uninitiated to how these mounts work, then buying new isn't a bad way to go, but will cost you more.
