throttlejunkey -
"P" refers to a "pinned" barrel - prior to 1982, S&W revolver barrels, in addition of course to being screwed solidly into the frame, were secured with a cross-pin at the top of the barrel just forward of the forcing cone area.
"R" refers to "recessed" or "counterbored" cylinders - again prior to 1982, S&W magnum and rimfire caliber revolvers (only) had chambers that had a counterbored area that fully enclosed the case head.
You can see photos of both features, along with other S&W identifying characteristics,
here.
Although both the pinned barrel and counterbored cylinder were probably not necessary for their intended purposes (preventing the barrel from loosening, and protecting the shooter in the event of a case head rupture, respectively) - after all, the celebrated Colt Python has neither - they are a reminder of a time when keeping production costs down wasn't the Holy Grail that it is today. Accordingly, and in spite of the fact that there is really no practical reason for it, many S&W enthusiasts (including this one) prefer to own P&R revolvers if possible, and that drives the value of them up a bit.