Giggle? Naaah, not really as I don't really know anymore about it than you do and you have it in your hands. With what you've described, it sounds as if you've confirmed what I think to have originated at the Georg Teschner/Wilhelm Collath facility. Have you attempted to insert a 20 bore cartridge? Does it drop in or experience some resistance at some dimension?

If could snap a pic of the standing breech which shows the firing pin holes. It's the front of the receiver where the barrels snug up to the action.

Kind Regards,

Raimey
rse