Thanks, OKiMarine for the interchangeability help. Now I will expand my parts searches.

Using your guide from the provided link both of mine are post 1924 because they have the later strait slide releases and diamond fore arms (1926?). They seem pre-1928-29 since they have the rocker shell stops, 3.5 inch tangs and trigger housing screws, not the pins. Both are marked as Stevens 520 (not another brand) so using your chart they are both 1926-1928 era versions. Does that sound correct?

The first one I got I did the usual barrel cut down at 19" and installed a tritium sight. Won an auction for a replacement stock with the round pistol grip (the original flat stock was broken in the usual spot where the wood meets the receiver on the right side of the upper tang). and mounted a barrel band sling mount with a 2-screw 1903 Springfield stock swivel. The thing just looks awesome. A true riot gun and fits in my 20" Army tool bag when broken down so it is very stealthy. I have some issues with it that I discussed on another thread.

The second one is currently awaiting parts. The beg problem is the slide arm was broken and brazed back on so I have to spend mucho dinero for a replacement along with an unbroken stock (I wonder why they all break off the upper right side?).