Well, well, cyclotoine... so I'm not alone, after all.

First of all, I have noticed in the past that many 22.2 stems will fit in some French steering tubes. It is always worth checking whether the inner diameter of a steering tube will accept more common stems. Sometimes it is only the top lock-nut of a headset which will present the "obstruction". If this is the case, it is easy to grind the inner circumference of the lock nut down a bit with a Dremel-type rotary tool and a small sanding drum attachment.

More interesting to me is that I too have had a French frameset pass through with a welded modification to the steering tube. I was truly shocked. I had removed the old headset to see if there would be enough length on the steering tube to put on a different headset with a somewhat higher stack height than the lower quality headset found on the bike. It was then that I discovered the suspicious "groove" cut into the rear of the threads rather than the flat file marks for the more usual "D" shaped washers typically found on French bikes. When I completely removed the fork, I found that almost half of the steering tube - including the threaded section, of course - had been carefully welded onto the top. The threads were indeed British!

This seemed a remarkably bothersome way to use a more common headset rather than to just find a replacement French threaded one - since this was apparently done when French headsets were still aplenty. Never seemed to compromise the strength of the fork, so no harm done... but how odd! I wonder whether this was more common than I ever would have imagined?