This from Wikki:
By 1970 HIWATT produced PA amps as well as 50 watt (DR504), 100 watt (DR103), and 200 watt (DR201) All Purpose amplifiers. Until this time Reeves had been building all of the amps in his garage with the help of his wife Daphne and Doug Fentiman. In early 1970 Reeves accepted that he could not keep up with demand and began looking for well qualified electrical technicians to be contracted.
Harry Joyce Electronics was selected and by mid 1970 was wiring chassis that were then sent to Reeves for final assembly and testing. This arrangement lasted until Reeves death in 1981 and Harry Joyce Electronics continued to wire chassis for the short lived Biacrown Electronics (1981-1984)
Could the "B" be Biacrown?