As I built the database for L.C. Smith shotguns, I had the opportunity to look at each entry in the ledgers. I do not think that the "R" suffix had any direct connection to shotguns with the Curtis forend release. I think that it was just a coincidence of when they were made. In most of the ledgers used during the Curtis forend era, either an "E" or "R" was entered in one column. It is well-known that the "E" designated a shotgun with automatic ejectors. So, what did the "R" stand for?
Over a couple of years, I called or emailed at least 20 individuals who had requested research letters for shotguns with an "R" suffix, and asked them if their shotgun had the Curtis forend release. While the majority had the release, many did not. It is my opinion that Hunter Arms simply used the "E" and "R" to separate the ejector guns from the extractor guns (regular?).
After the Hunter Arms Company failed and was bought by a group of Fulton citizens, a new ledger was started. The "E" continued to be used for the shotguns with automatic ejectors, and use of the "R" was abandoned.