https://www.midwestgunworks.com/page/mgwi/prod/60426
Benelli Recoil Spring Tube Plug. This tube plug attaches to the end of the recoil spring tube and retains the follower and action spring inside the tube of your 12/20 gauge shotgun. This plug fits Benelli M1, M2, M3, SBE, SBE II, Sport II, Supernova, Ultra Light & Montefeltro shotgun models. The plug also combines with the stock bolt washer and nut retains the stock to the shotgun. This plug is made to the same high standards in craftsmanship and materials as you would find in any other Benelli firearm part. This is a great replacement for a worn or damaged part, or have this as a spare to keep your Benelli shotgun in top operating condition.
This looks like it threads into the recoil tube, then takes a plate and a nut to keep everything together?
https://www.midwestgunworks.com/benelli-m2/locking-plate.html
Seems like there are some options....