Thanks to all, but just before I started to take the gun appart I tried to manually depress the shell stop and it worked. It was hard, but it moved. Whether this was because something that had jammed it had loosened, or my finger nail had grown longer and worked better in pressing the stop to the side, or the new use of a Bowflex gave me a better purchase, I don't know, but I can now unload as intended. I swear that sucker wouldn't move with all the force I could muster; it wouldn't even flex. But now, with some effort it is fine.
HC