Hi,
It would have been German made. According to Rainwater, BB imported from Bek before WWI and purchased their entire stock afterward in 1919. Binder did manufacture jewelry, but used their own "BB" trademark on what they made.
German manufacturers seem to have had complete freedom to mark items however they chose when the product was intended for export. Technically, by American law (McKinley Tariff Act, 1890), it should have been marked "Germany". Judging by the great amount of foreign made jewelry, silver and other metalwork that was imported into the U.S. - that is not stamped with country of origin's name, either there was not much enforcement of the law or paper labels, hang tags, etc. met its requirements.
Regards, Tom
ps. More info on the McKinley Tariff Act
viewtopic.php?t=14366.