If I understood the OP correctly, the P and R are on top of the chamber. This barrel surfaces ever so often, but so far I know of no source of the barrel. There are a few barrels from 1916 that does not have the H for Frank Hosmericon, as he had been transferred from Colt to San Antonio Arsenal for a short period. During that time the top of the chamber had a P and the end of the barrel hood had an S for W. E. Strong. Possibly for a brief period Frank Hosmer was taken away from his duties at Colt, and someone whose name started with an R did the provisional inspections.
The Springfield barrels have the S and P down on the barrel lug.