Here's a link to a PDF on GGCA's 2007 article on Bernhard Merkel which states that some of his examples had BESMU: http://www.germanguns.com/journals/newsletter3.pdf . He also used a mark similar to the Gebrüder Merkel stamp of a "GM" in a shield(either Gebrüder Merkel or the "GM" in a shield was filed under #331084 in 1902 I believe) which was "BM" in a shield. So with that I'd venture he also used his initials.

Kind Regards,

Raimey
rse