Clif:
So what's the issue? The gun was probably ordered and regulated to shoot a 7/8 oz. load of some unknown shot size. Only in the US is 7/8 oz. NOT a prevailing "normal" for a 20-bore field load. That much shot is plenty for 98% of all shooting.

Not to say the gun won't handle 1 oz... but it won't be regulated for it.