I think this "B" could be the makers mark of
Ludwig Friedrich Brugger from Bern
The city mark from Bern is a "bear"
Brugger was working in 1810
and he didn't juse the mark "12" but he jused the mark "XI" crowned
I found this "B" in the books
Altes Berner Silber im Schloss Jegensdorf (1949) and in
Die Goldschmiede der Stadt Bern, Marie Roosen-Runge (1950)