Any 2 3/4 ammunition you desire or is proper for the gun. When Parker and others chambered 2 5/8 they intended the gun to use longer 2 3/4 hulls. I think the reasoning was the hull would partially open into the forcing cone and provide a better gas seal as the fiber wad moved from hull to bore.

Parker hang tags on new guns said, Chambers 2 5/8, next line said, use 2 3/4 shells.

Pretty clear.

When an old man dies a library burns to the ground. (Old African proverb)