Originally Posted by jccaclimber

There are plenty of ways to measure fasteners. You can use calipers for the diameter and pitch. You can use a thread gauge for the pitch, get close with a ruler, and make some reasonable assumptions to get the rest of the way there. You can compare to known part either from your stock or at a local store, either hardware or bicycle. A local bicycle co-op would likely also have the parts for even less, albeit used. As to if they allow work for profit (even minor) depends on the outfit.

High odds that it’s metric unless British of a certain age. Once you’ve narrowed to that you can probably get close enough on axle diameter with nothing more than a ruler. Easier if the ruler reads in mm, though estimating to the nearest 1/64” and doing some math will get you there too.

I make no claims as to what the old French bike is, even once you have a unit system figured out, though the “see what fits” solution will probably work. I’d try that at a shop though, at least where I am the local hardware store selection of metric fasteners is rather limited to put it politely.

Edit, those are older than I was thinking when I read your country of origin comment. “Test for fit” is probably going to be fastest unless you want to get a thread pitch gauge. The good news is that a cheap one is $10 on Amazon and will last a lifetime of home use.