I think the convertable pedal/ knee controller came later on (mine is for a button bakelite controller). The earlier version had a sort of ceramic container for the carbon stack, it works rather well. It can be sorted out, rewired, cleaned, adjusted for speed, etc. I kept the original on my machine and I like it.
I don't know about switching to and from a pedal; I know some three pin socket machines can convert to the pedal and mains cord and keep the knee controller untouched (bent wood case model). Others report back they needed to rewire the machine to get the machine to respond at all when turning to pedal controller.
I envy you the No 40 cabinet, the veneer and finish are so nice on them. I would keep it as original as practically possible. If your machine hasthe original woven cotton covered wiring I assume it's very stiff and the rubber insulation under is crumbling away. If the wiring looks all right I'm sure it's been rewired all ready. The original insulation on machines from the 1920s and 30s are always in a terrible state.