Got to love old Mossberg pot guns... Back in the day, one of my hunting pals had one of those, and I grew up around a Mossberg bolt action shotgun that brought home a lot of venison and a truck load of ducks and bunnys as well. It is semi-retired now, residing comfortably in the den... I wish I still had the H&R top break .22 revolver I used to pack around with it.
I think you will find that the 'disc' in front of the trigger group is a coin slotted screw... that holds the stock on.
GTivan, I believe that the "disc" in front of the trigger simply is the screw that mounts the action/barrel to the stock. They probably made it with provisions to tighten the screw by hand, making it easy to tighten while target shooting. Makes sense to me!!!! :lol: Bob
Sometimes we forget just how accurate most of the older entry-level bolt-action .22LRs were. Even as humble as they were, the barrels and actions were still hand-fitted into the stocks, and they will usually out shoot all of the entry-level .22s of today.