I still can't figure out how to post pictures directly, but here are more photographs of the various proofmarks including the marks on the receiver:https://drive.google.com/drive/folders/12Ld1C6_t8ervXIjKbg-_dfhgT8ByIeRf?usp=sharing
Der Ami, I doubt it was marked .410 by mistake. The proofmarks indicate that the barrel was rifled originally, and it most definitely is not now. Plus the stars on either side of the "16 x 410" definitely do not have a Germanic look to them. So I am going with Ellenbr's theory that it was originally meant for a rifle cartridge and was bored out sometime by a USAnian gunsmith. Thank you all for sharing your knowledge.
