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Hello 3-speeder,
Thanks so much for sharing this wonderful find!
Great job on the photos as well.
As Thor cycles were produced in Torhout, Belgium; do not know years of operation.
It is likely built to BSC/ISO standard with regard to tubing & threading. Some Belgian machines were also built to french/metric standard.
This one looks like it might be near to 1954. Best dating aid seen is the cycle's Huret shift lever which entered service in the early 1950's. It is possible the bicycle left the factory as a derailleur three or four speed and was later converted to 5V. What make is the gear block?
Is chainset Acier Diamant?
Wheel rims look like they might be Alesa.
Looks to be mostly original. Non-original fittings include rear mech, saddle, wheel reflectors, pedals, brake levers, second rear reflector & saddle pillar.
Mechanicing tip - saddle pillar has been replaced with one too small. Note shimming. Seat binder ears of seat lug have been crushed down. I'd want to remove the pillar and check the true size. Would expect something like 26.2 or 26.4mm. You can open up those binder ears, gently. Then locate a correct sized pillar for the bike.
Here are some images I saved of an earlier example, probably from the 1940's:
Thank you again for sharing this wonderful find!
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