Planning to buy a Chief Mount for a JVC RS-2. What is difference in the various key options as designated by the A, B and C options for the RPM 184 mount?

This doesn't really answer your question, but I believe those are security mounts (as compared to the standard ceiling mount RPA series). Are you concerned with someone stealing the PJ?

Don't know if you have already bought a mount or whatnot, but the only difference is the key that locks the mount. I bought one of the RPMA mounts, not for the keyed lock but the microzone adjustment. This was for my first projector, but I have to say that the mount was a pleasure to use.

I have found in my long career that for projector mounts, the Chief's are the best.