...and I should have added that the stem is also smaller diameter, because of the headset. (Not the 'bars - they're probably 25.4mm). I'm writing from memory now, but I think French steerers were 21.8mm instead of 22.2. Both those could well be wrong, though - it's a long time since I've needed to remember those!
I had a Peugeot road frame in the 80s which must have been one of the last to be made with French threads; it was a right pain. Even then, I couldn't get a stem to fit the bike (although I didn't have www.retrobike.co.uk
to help me!) and ended up having to grind off some of an English-type stem (which isn't a great idea, really). I eventually changed the forks.