I've heard of a couple of MKIIIs with a PLY prefix SN added to the barrel. It's not an original factory SN, on the MKIII they were stamped into the wood buttstock. My guess is that they were added in Canada but don't know by whom or why. Will try to remember to take a look in my copy of The Ross Rifle Story when I get home tonight. Seem to remember something about the letters PLY being in there.
You certainly got a lot for $25! Me, I'd of tracked down one of the numerous bubba'd MKIIIs out there with complete actions and a good bore and between the two piles of parts would have gotten a nice MKIII, worth at least $500. That original, full length stock was probably worth $200 or more, they're impossible to find along with the upper band.
As to the rivet on the bolt, not all MKIIIs have 'em. I own two ex RCA MKIIIs and a British Home Guard MKIIIB and none of them has the rivet. Someone posted two excellent photos showing a properly assembled bolt in one and an improperly assembled bolt in the other. Made it easy to tell if your bolt was safe to shoot. Printed them out but can't remember where I ran across it for the life of me (I'm at work and don't have access to my stored bookmarks on my home system).
The problem with the rat race is that even if you win you're still a rat!