I bought one of their Mk IV (pistol-gripped, but not folding) stocks for my tanker M1. Aside from a gap in front of the floorplate (since it was made for a mag-fed rifle) it fits perfectly. I took the metal bits off my regular M1 stock, slapped them onto the BM-59 stock, and put it right in place.
Shipping took about a week, IIRC. The only surprise for me was that it came unfinished, so I had to get some finish oil before using it.
Like natedog said, the folding ones were intended for BM-59s. They should fit on M1s with no problems. Oh - do make sure your M1 has the later-model stamped trigger guard. The earlier milled one (distinguishable by a circular hole at the rear) is too big to fit in front of the pistol grip on these stocks.
The experimental M1 stocks were metal underfolders with no pistol grip (I have a picture on paper, but no scanner...). Unsurprisingly, they didn't survive past the experimental stage. The BM-59s, however, were successful issue rifles for the Italians (and a couple more minor nations that bought them).
Personally, I wouldn't get one of those folders for an M1. I've handled them on BMs at shows, and I think they make lousy cheek welds. If I want to make my M1 smaller for transport, I'll just take to stock off, which hardly takes longer than unfolding a folder anyway. Lastly, I can't see myself ever wanting to shoot it with the stock folded. So, (and considering the ~$180 cost difference) I just bought a pistol-gripped solid stock (it also has a storage compartment like the M1). FWIW, the one I got has the same LOP as a normal M1 stock, but the pistol grip puts makes the LOP seem shorter. Some people who've shot mine have mentioned that it feels too short. I like it, though.