Never saw a parkerized M-15 myself. All those I saw were blued.
We got a shipment of new ones while I was at Panama. They had the least polish of any blued gun I've ever seen. Not parkerized, but obviously not much polishing of the metal before bluing. Certainly not like the ones we had been using.
And those new ones had rough actions. We were surprised at their lack of fit and finish, because the S&W had been so well made, even for military models.
They were probably made in 1976 or 1977, because I recall our 24th Security Police Squadron getting them in 1978.
I was offered a new one, being an E-4, but hung onto my old one. It had the finest action I'd ever seen. Popping the sideplate off one night revealed that someone prior to me had gone in it and polished all surfaces. They knew what they were doing, to be sure.
Its finish was thin, from holster wear, but the bore and chambers were mirror-like.
Even now, more than 30 years later, I wish I could have purchased it and taken it home with me. I almost weep to think that when the M9s appeared, it likely fell to the cutting torch because it looked worn.
It wasn't worn ... it was just broken-in!