Getting closer to figuring this out...
3 X 5 card resulted in more or less the same as the feeler gauge
Pic 1 is a 20 ga sold to me as a 2 1/2 chambered gun
Pic 2 is a 12 ga which I thought was 2 3/4 but probably 2 5/8 the way the markings fall compared to the 20 ga
I checked my 16 ga and the markings fall the same as the 20 ga - so that one must be 2 1/2 and currently cannot find shells for.
Clays ammo has them in a container off the coast of Florida.
My plan is to use the light load 2 3/4 shells in the 12 ga
RST 2 1/2 shells in the 20 ga
And continue searching for the 16 ga 2 1/2 shells.
Would love to find shells for the .410 that was just offered for sale, but that may have to wait a while