Maxrad,
Trigger housing screws and a straight slide release puts them in the 1926-28 range (with some overlap w/1925). Stevens didn't make clean yearly breaks in their design changes so that's the best estimate i can come up with. I am building a serial number database organized by design change. With around a hundred observed S/Ns so far, the numbers support my evolution model of the 520 (and 520A/620/620A). I'd like to add your shotguns to the list if you don't mind.

Sounds like a nice custom 520 riot, post some pictures. I'm working on a shorty 520 with a cut stock (a legal version of the Bonnie & Clyde 520 whipit gun).

I haven't had the slide/action arm disconnect problem that others seem to have had but I have seem lots of them w/ that broken piece on the right front of the stock. Just picked up a 520 20ga w/ a repair in that very area.