All this trials RE chatter prompted me to dig out Don Morley's series of articles for Motor Cycle Sport in 1978. Unfortunately mostly post-war, but fascinating none-the-less.
I don't know much about the subject, but I can say that the "BTH racing magneto" fitted to trials bikes of this era was the same "TT Magneto" as fitted to race bikes. I was surprised when I learned this, but the major feature of interest is the waterproof-but-vented design, and presumably some extra quality control. As well as RE, the BTH TT Magneto was fitted standard to things like the Norton 500T Trials bike. The model was something like a KD1 C6 - some special ones even used magnesium alloy bodies.
There are various repro Dunlop Drilastic saddle tops around, but quality is variable. The best ones seem to be made for the German bikes, but they often have German writing cast in.
My racing Rudge used both BTH TT magneto and Dunlop Drilastic saddle.
Leon