Although it was called Birmingham Sound reproducers, non of the factories were actually located in Birmingham!
There were three factories in Old Hill/Cradley and one in Stourbridge, which are all in the Black Country, there weren't even any Brummies working there that I can remember of! Most were good old Black Country folk, which back in the day was in the county of either Worcester, South Staffs, or Warwickshire! (It had been individually all three when I was a youngster) It certainly was Never Birmingham, saying that would probably have caused a riot!
I also seem to remember one factory being in Ireland.
I worked at both Garratts Lane and (Stourbridge temporary) between 1975 & 1977, happy days..