They look to be pre 1875 Birmingham black powder marks to me. If you contact Smallbore on this forum he is in the gun trade with experience in exporting and an expert on these things. I'm sure he can help with some form of letter that can confirm for you and provide suitable evidence. Failing that e-mail the picture to the Birmingham Proof House and obtain evidence from them. Their say so should be all the dealer requires. Lagopus.....