Are you absolutely positive they are B's back to back, or could they possibly be cursive elongated C's? I know I just recently came across a Vintage Cartier watch that had the sapphire cap over the pin and elongated C's as in the Cartier logo, their writing is distinct and they do their watches in 18k gold, as well as their trademark sapphires over the pin. I haven't seen it, but sounded like a lot of things in common with a Cartier watch. Good luck !
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Mirrored B Solid Silver Watch Logo by: Bob Brinklow
Mirrored B's with a Crown on top, solid silver watch but who's logo is that?
I know the crown above B (not Back to Back) is a Netherlands Tax mark on Imported SILVER - ca 19thC
Sorry Edward, i can not be of more help, but without even an image to go it could take the researcher days of research and still not arrive at an accurate hallmark determination.