The difference between Model 10 and Model 12 stocks is seen at the top as a gap between the side of the frame and the "horn" of the stocks. The major thickness of the M-12 frame is .080" thinner than the steel frame models, while the grip portion of the frame is the same. This results in a .040" gap under the horn. Other than this they are the same size.
Standard K-frame stocks can be used on the model 12 by sanding the .040" off the back of the stocks by laying sandpaper on a flat surface and then rubbing each stock half over the sandpaper. It is usually referred to as flat sanding. This is a lot easier than finding original Model 12 stocks! I don't understand why you referred to those Pachmayr grips as original, they are not. The various rubber stocks are very often seen on Model 12s because they don't have the problem of the gap under the horn. You can also get reproduction hard rubber style stocks for it from N.C. Ordnance. They make the gun look like the first M&P Air Weight guns when they were introduced in 1935, and there is no issue with the gap since they do not have the horn at the top!
The gun looks like the finish is in good shape. Be aware that the anodizing on the aluminum frame can wear more quickly than a steel frame if the gun is carried in a holster much.
If you have inspected the gun and it is sound you are in good shape. Be aware that some wag will be along shortly to tell you about Model 12 frames cracking!
Some people seem to believe this is a common occurrence, it is not, so just ignore whoever does this. While the Aluminum framed models are seen to crack at the bottom of the lunette, for some reason people pick on the Model 12 incessantly, while rarely saying anything about any of the many other models that are Aluminum!
You should enjoy that gun, believe me, I have owned five of them and still have four. They are a joy to carry and much easier to shoot well than the J-frames because they are big enough for someone with normal size hands to hold onto.
I would show you a picture of my M-12s, but I don't have a photo program on this new computer.