I noticed (finally) that the grips on this model aren't removable and that the gun seems to be a lot slimmer than the other MK III's. What is the advantage of the molded grip panels and thinner gun, if any?
The original Mark III 22/45s only came with the molded grips in the frame and were definitely slimmer than the original 22/45 model.
The advantage in this is typically seen for female shooters and juniors, who generally have smaller hand sizes than most men. The smaller girth frame allows them to better enjoy shooting a pistol designed more to their size. Ruger listened to the consumer complaints, and paid attention to owners who were modifying the frames, to take 1911 grip panels by shaving off the molded ones and inserting 1911 grip bushings. This was happening so frequently that they decided to exploit that market and manufacture a grip frame that would accept 1911 replacement panels and thus the 22/45 "RP" was born.
@ Rugerfreak - The "P" designation in the P512 model number signifies a 22/45 polymer framed pistol with a five and a half inch long barrel. The metal framed pistols have the "MK" or "KMK" designation before the length number, and is for regular blued carbon steel or stainless steel frames.