I would guess that's Bubba's handiwork, albeit a very nicely done conversion. They even installed the matching handguard! I agree that it definitely looks like a Sporter stock with the slab sides and rubber grip at the thumbhole. If you wind up getting it and can strip it from the stock, look to see how and where they opened up the bottom of the receiver to allow for the AK mag to seat. If they milled it and took the time to reblue the in-the-white areas, bubba was a true craftsman!
I'd assume that it should cycle just fine. The gas system on a M59 is the same as a T56. If it was an M59/66 then you might have issues. As for feeding rounds, the stock determines the position of the magazine, so as long as the receiver has enough clearance for the magazine to seat all the way in, it should be in the correct position with the feed lips angled correctly. I would assume whoever built it tested it out and tweaked things as necessary if there were issues.