Re: Old gun, loaded in the front. (sorry for the english, i am danish)
U.S. law requires that commercially imported items (other than cartridge firearms) be marked with the "country of origin" or "COO". The "Japan" mark shows that the gun was imported commercially into the U.S., not a vet bringback or an antique (antiques are exempt from the law since the mark would devalue them).
So is your Grandfather's gun "authentic"? Yes, it is an authentic gun. It is just not an antique gun or a valuable one.