I know this is long after you made your post on the M12 with +P ammo. I just bought a 4" SQ. butt in NIB condition. Curious about it, I wrote to S&W and they said NO +P. My revolver is D657XXX so I assume it is '73-'74 based on your sample. I located a M12-4" ROUND BUTT, this afternoon. It is the first one I have seen. Why it calls to me I can only guess, but it feels very good in the hand.
It is too bad that the alloys now in use were not used in the M12 series. I like the +P ammo.
There seems to be many NIB condition M12 revolvers available now. Why, I can only speculate, but a local shop has several.
Now the issue is should I even use this fine condition gun or collect it. I don't like collecting, I like using.