Boulder Bicycles is the successor to the French builder, you get new bikes custom made by them under that name now... made in COLORADO. ask them,
they kept the top of the fork steerer switch that made those brushes needed.

site link in #3 should help.

But I dont get the reason the BB dynamo needs a switch
purpose seems not thought through
... Sanyo BB generators switch on and off

by lifting the dynamo drum off the wheel, no switch needed...

Its tire driven .

the custom feature back in the day was a shift lever
on the top backside of the seat tube operating a cable down
to engage or disengage the Generator without stopping..

the setup now is Schmidt Hub Generators , then the switch is either on the headlight itself. or if you were buying one of those $6000 + Full custom bike frames
you can get the top of the stem switch ..

NB the frame fork and rack at 6K+, not including bars wheels components.
saddle, tires.

I used to have one of those Sanyo Generators [still do it's in a bin around here]
I found it easy to trip the spring catch lever with my left toe,
with out any fancy brazed on lever .. just had to stop when I wanted to turn it off.

I'm Never in so big a rush , I cannot manage that sort of task.

Sanyo , big electrics company that they are , are also in the hub generator game , I understand they make the parts for Shimano and all those other Taiwan Based brands: SRAM and Sturmey Archer Sun Race.

then you take the electrical load off the hub generator my switching off the lights,
it keeps moving the coil of wire around in that magnetic field,
but the resistance goes down..