FWIW maybe it is premature senility on my part, but I don't remember saying that, not quite!
IME French and British spindles are usually slightly different diameters, such that you can use a mixed combination in some cases, provided you are happy to tolerate a poor quality fit between the parts (which you shouldn't be unless desperate). I suppose that slightly worn parts might go together in the other combination too, and of course cranks can have been made differently or modified after manufacture too.
Strictly speaking BS cotter pins are 3/8" dia (9.525mm) I think but 9.5mm is close enough bearing in mind that any you can find to buy new are such lousy quality.... 9mm cotter pins in most French stuff. I've heard tell of 8.5mm cotter pins but have not had the 'pleasure' in person as it were.
BTW if you take an old French BB, often the shoulder distance on the spindle is smaller than many other BBs, such that most BB spindles won't work with the original cups. However it turns out the shoulder distance is quite close to that found in 'Neco' three-piece BBs, which often allows you to fit a cotterless chainset to a French-threaded frame, using the original BB cups with a Neco spindle.
cheers