I checked the shell latch very carefully and wouldn't change it, as it appears to work very well, but with minimal shell contact.
The shotgun is in very good condition. Parts don't appear to be worn and the fact that it functions every time with rounds in the magazine, but only malfunctions on the last shot is nothing I can cure without a bunch of new parts to try, so I'll give up on this one and let the guy send it to the factory or to an authorized repair station.
Thanks for all your good advice. I learned quite a bit about this shotgun model.
People are like rifles. Some are tried and true, having great eyes, personality, and fun to be with. Others never seem to hit the mark with you. Still others go off half-cocked. Still, it's nice to know most of them.