Harold --

This might work for you.  There is a Dutch word *bouwmeester*, that
means 'architect' and/ or 'surveyor' (see Sewall's Dutch-English
Dictionary, various editions between c1700-1750).   In this same
dictionary, *bouw* is given for 'building' -- and of course *meester*
is 'master'.

I do not know what this would be in German -- but the languages were
close.

If this is the meaning of BM, it suggests that the men who had this
after their names were the designers / architects of the building.
Perhaps that was their only role (though they may had a hand in or
directed the construction, etc.), while the other names were the names
of the men who built it.

Ruth Piwonka
Kinderhook