.... oh the stories to tell about vintage items going in the bin or sold off for peanuts, only later to find someone selling them on Ebay for the price of a house extension or small car.
Agree that it all looks close clearance. The dates seem to match up - handlebar mounted bottle cages etc. Wasn't 531 pretty much the top end lightweight tube set back then too? Simplex shifter and what looks like a TdF rear mech in one photo.
I'm not familiar with the 26" x 1 1/4 size:
https://sheldonbrown.com/harris/tires/597.html (thank god - and you say French bikes are a PITA
)
There's most certainly a rear wheel missing; I'm guessing another Mavic Sprint rim but laced to a hub for a 3 or 4 speed freewheel. The Sprint rim with the fixed gear hub was used in winter for some hard winter miles. The owner must have been a serious cyclist and you can tell he liked decent kit.
Without a catalogue / advert from this period it will be hard to know for sure what wheel size it was designed for.
Does
@Peachy! know anything about this CB model?