Hi Non-Fixie,
I'd like to ask you how the LeSimplex shifter looks
See my picture
This because it would help dating your rusty french bike
From the equipment that looks period
So far it looks post wwII to me but this may help to narrow the period
Good for you, now you can find 650B tyres at D4 shops, at a very low price. Unless you want to spend for names.
The picture is from an intriging Belgian randonneur that I bought in last quarter (Too many new bikes and no time)
The shifter with the S is more recent than with "Le Simplex made in france" and that S marked shifter is already visible on a 1949 Simplex catalog visible on DisraelliGears
I assumed my randonneur being older than that.