I think that the R suffix simply means that that the shotgun has extractors rather than ejectors. During that manufacturing period, one column of the ledgers contained an "E" or "R". Earlier and later, nothing was recorded in that column other than an "E". I suppose that if you had "E" which was used from the start, there was no reason to use another letter to indicate extractor. I think that the "R" stood for "regular" or some similar term.
To test this idea, I checked with 15-20 people who had requested research letters for shotguns made during this period to see if their shotguns had the Curtis forend release. About half had the Curtis, and the other half did not. But, all had extractors.
Drew, this may be one of the things we never know, but I am interested in learning if there are any unaltered shotguns with the letter "R" as a SUFFIX to the serial number that have automatic ejectors.