Well, a"day or two" has turned into 8 MONTHS and I finally got my spencer back yesterday. I excitedly opened the box to discover what the "OFFICIAL" repair for this problem is. According to the work sheet that came back with the gun, they "adjusted & tuned Upper Block & stop Plate". Upon inspecting the repains, I observed that this consists of hammering and filing the worn-off portion of what the parts drawing calls the "trigger guard" back into shape. This actually leaves LESS contact surface between the upper block and trigger guard than they originally had. After digesting this little bit of info, I decided to do a test run. While working the action briskly enough to eject empty cases, I was able to cycle the action 13 TIMES before failure of the stop.
After a "calming down" period, I tried putting a spacer between the blocks as suggested in this thread. Unfortunately, due to the geometry and relationship of the blocks and inside of the action of my gun, it would not work. I then phoned the "fella what done the work" and discussed how to remedy the situation. He is now going to send (after HE gets it in) a new trigger guard for me to install. So that's where we stand right now. More -- HOPEFULLY -- to follow in a few weeks.
Gus