Oh, there's a bunch of differences. Yes the original FP switched only the "hot" signal lines to select the source. The level controls had quite a number of variants, especially if you count the ones popular on the Forum that never became Bottlehead kits but were widely used, and the various approaches to reducing the gain which was excessive for some power amplifier/speaker combinations.
Also the chassis was grounded to signal ground at some point; I'm not sure when. We were still learning about grounding, ground loops, and shielding. Well, honestly, we aren't done learning yet!
The design started with George Wright, then for several years the modifications were done sort of collaboratively with several people including various Forum members. When I re-did the design as FP-III - 2005 maybe? - I changed the operating points of the triodes and introduced the Sweetest Whispers topology - yet another level control. As I recall, I was over at the Schmalle's that weekend and the idea occurred to me during a morning run with Dan. Since then, I did the volume/balance control in Stereomour, and later Paul Birkeland made the BeeQuiet coarse/fine switched attenuator which is currently our best.
I still run the FP-III in my own system.
