I recently bought a Manton 20 bore non-ejector which has a 4 digit serial number and appears to be in very good shape. The gun has 2 1/2 inch chambers and the proof marks show "7/8" rather than "tons". I assume the 7/8 is a reference to the shot charge?

I have some 2 1/2" Gamebird brand shotshells with a 1 oz. charge. The label on the box says they are for use only in guns proofed for at least 3 tons. My question is - do you think it is safe to fire the 1 oz charges in the Manton (assuming of course that it is otherwise in good working order).

many thanks