Gentlemen: I just read through most of the archived threads on the "R" suffix. Now to add my bacon grease to this fire! I just purchased a 20 ga. OO, SN 41XX. I haven't received the gun yet, but one of the photos showed the barrel flats. The markings included the SN, the "OO" grade under the SN, and "R" stamped a good distance the right of the SN. Seller indicated all SN's matched, frame, barrels, and forend. The gun is equipped with EJECTORS, but the gun has no "E" after the grade. It has a roller latch on the forend, but the escutcheon for it is plain, narrow, and parallel-sided (not banjo-style).
So, what can we surmise? Serial number lookup indicates 1912 completion date. Featherweight frame (screws in the front extension of the lockplates). It appears that the gun was originally shipped with extractors, and returned to have ejectors retrofitted. It would seem plausible that the "R" was stamped on it at that time.
It also appears that this gun was produced prior to the Curtis Release (OO grade, 1912). When I receive the gun, I should be able to confirm that the forend wood is matching, and if it is inletted for the Curtis hardware. Seems unlikely. Photos to follow, if I can figure out how to upload them.