Experiance,
I have a gamecock and have hunted with it quite a bit. Funny, your gun has more markings on it than mine does (I don't have anything that resembles a case length either. Mine is a XX so I guess that makes it a 65 vintage.)
I have had very good luck with 2 3/4 bismuth on creatures that required non-tox. For anything larger I went to the short Remington magnum with a 1oz buffered load of 5s. The pattern was about as clean as I could have hoped for, with both tubes patterning tight to the listed IC/ mod chokes. BTW, the mod chokes closer to Imp-mod.

Both of the above loads pattern very well, which I would put before dropping in an extra 1/8oz of pellets and the associated recoil, etc.

Cheers

Tom

Carbonation without fermentation is tyranny