B & M were originally based on the south coastal region of England and started making their own banjos after gaining some popularity playing them. They later moved to London and it seems they, like many others, had people like Temlett and Windsor make banjos for them.
This is a fairly late addition to the range, though still, I believe, pre-2nd World War. The quality of finish suggests J G Abbott as the builder, and particularly the scalloped tension ring, Clifford Essex as the the hardware supplier.
I have seen a five string version on the Web but not another b/mandolin.
The tone and volume are amazing, perhaps as a result of the all metal body and adjustable arch top tone ring.
If anyone has further information on this type of banjo please let me know.