You know if you have a felt wad, 10 gauge shell and 12 bore it will work just fine. Might have been ordered by a live bird or hunter with very special ideas about what would shoot a real tight pattern. Bore diamaters are all over the place on doubles. Add honing to get out pits and you never know what they started out as in the first place. There was also the chambering that was for what we think of as non standard shells before they became a standard size.
I wonder how well the gun patterns with felt wadding and lead shot? I have seen some real strange combos in my travels. Two rear beads, one rear, one middle and one front bead on the same gun. You never know. 10/12?
