I think Kennett's reference to R powder likely comes from Baron Engelhardt, whose work he was basically updating. Engelhardt's original article shows the PM proofmark and says: "For barrels proved with smokeless R and M powders, adopted May 14, 1898. But it does not show a PR proofmark. He also shows only the PJ proofmark when referring to both J and S powders, which were adopted together on March 30, 1896. Looks like something slipped in the editorial process. Kennett's later article does show a PR proofmark.