I have a 74 Series III, bought it new. When high revs in reverse (usually getting un stuck) it "pops" out of reverse. It's always done it, not really a big problem cause I don't usually need to use hi revs to get unstuck or have the need to go fast in reverse but I was just wondering, now that I can connect to other rover owners, if this is normal.

on an old box yes ,may be improved by renewing detent spring and setting rev fork ,but mostly a full rebuild

How violently is it coming out is if just sort of flopping out then renew the detent springs. There held in by 2 l shaped brackets and a brass nut on the top gearbox cover, nice easy job.

If its being forced out then there will be a fault with the box, you can remove the selectors and have a look inside for any damage to teeth etc but that would be a rebuild job.