I can't remember exactly which bikes now, but I too have sometimes seen late 70s French bikes with 22.2 steerering tubes, but metric threads and 22.0 top nuts on the headset. I recall Sheldon Brown once mentioned this too. I am really not sure if this was deliberate or whether it was a matter of tubing availability at the factory. When French bikes began to switch over in the early 80s there was even more confusion. A bike could have a BSC bottom bracket and a 22.0 steering tube, and they still used the narrower metric tubing diameters as well. To be honest, I've gotten in the habit of testing a spare 22.2 stem in the steering tube before even putting a new headset on, just to make sure I know what I'm actually dealing with for every French bike I handle. Great luck if you find a larger diameter steering tube - gives a lot more options, and its easy to just widen a lock nut a bit (couple twirls with a Dremel tool and you're there).