here's some more on FB, from Chuck (read the archives!) Schmidt of the CR list:
"The company name is F.B. and stands for Fratelli Brivio (Brivio Brothers
in Italian).

The hubs are stamped "Made in Italy" (even the Simplex-branded hubs are
stamped that way; there was Simplex Italy in the early 1950s making
Simplex derailleurs in Italy, and Fausto Coppi won the '49 TdF on a
Bianchi with Simplex derailleurs).

Tullio Campagnolo bought the rods for his Cambio Corsa from F.B. in the 1930s.

F.B. supplied branded hubs to many Italian bike and parts manufacturers
(Bianchi, Legnano, Maino, Campagnolo, Simplex Italy, etc.)

F.B. was still in business in the 1960s and selling hubs under their own
name after Campagnolo came out with their one piece aluminum alloy hub
in 1958."
Steve Maasland states that FB also made cranks, but nobody there says they made frames...
check out the threading on the BB and report back, when you get a chance