You might try pushing inward on the magazine cap, and if the pin hole lines up with the bushing, insert the locking machine screw and carefully thread it into the female threads.
I'm more of a Model 12 man, so this is a guess, but I once had a M31 12 gauge- 28" Mod. solid rib, nicely stocked, fit me well, but as I started shooting with Model 12's at age 11, got used to the front of the trigger guard safety of the Winchester, so when a late season rooster pheasant erupted at my boots, and I had the M31- I fumbled for the safety, and when I finally got it moved to fire position, the big rooster was topping the trees, on some State owned land here in MI-- I hit him, but without my Lab, would have never had him in the game bag-. Sold it to a DU pal shortly afterwards- smooth as silk action. I am of the belief that the barrel take-down on the M31 may be similar to that found on the Ithaca M37's- RWTF