The gear-driven ones do lack the R, but that alone doesn't mean they are (or are not) the correct ones for the gun. Supposedly, the gear-driven slide, thought to be an improvement, wasn't, and it is believed that no arms were actually issued with it.
Mine, 318991, issued, per SRS, to my (VERY distant) cousin CPT W. C. Manning, 23rd Infantry, was returned to SA for "problems with the sights" and now has a standard "R" Buffington, which does appear to have slightly less wear than the rest of the rifle. No way of knowing what it was originally issued with. It bears dual cartouches [SWP/1885] & [SWP/1887] signifying the rework.