The links I gave (which contain the same info as Barry's links) are straight from the horse's mouth - the BBC's own technical archive. Plessys were used in the LS5/1, changed to Goodmans when turned into LS5/1A
The mention of the Plessys were before the photos of the actual speakers were posted up. All the Plessy info in the thread appears to be true and confirmed by BBC documents.