James, When Arthur P. Curtis first applied for this patent in 1913, he was employed by Hunter Arms. By the time it was Patented it was now 1917. At some time in 1917 Curtis left Hunter Arms and went to work for a few other gun makers.
It is quite possible that he might have stated in working for Hunter, that if he left Hunter Arms Co. they would have to pay him royalties, but I do believe it was an expensive part to manufacture and the new owners were trying to cut costs.
Curtis did return to Hunter Arms Co. in 1940 as superintendent, a short term as he passed on in 1941.
In my files of guns with the Curtis for-end fastener and from some research, all seem to be Featherweight guns from 1912-1919, that I have seen. Also, from what I have seen, none were marked FW, just the serial number and then "R". The Regular frame guns with the Curtis that I have seen and own, do not have the "R" suffix, and on mine do not have the "R" prefix.
It would be nice to hear what others have stamped on their Curtis for-ends in this time period. Maybe we can get an answer as to the "R" suffix.