CJN,

I am glad to hear that someone else has a rifle with a similar marking

As far as I can tell, everything is 'R' marked including the bolt and its components. It genuinely appears to be an original rifle except for this odd stock marking

I was able to pick up a copy of 'United Statesicon Rifle Model of 1917' by CS Ferris, but I have yet to really look through it. At quick glance though, I did not see anything similar to these markings that are in question