you have a vintage tandem with a 1 1/8 headset? My impression was that the size only came in the early to mid-1990s, and then got quickly overtaken by the threadless headsets.
Mavic did their last headset design in a 1 1/8, model number was 316. One would think that would be a logical place to look for a french threaded headset, though the instruction manual I have only lists a 28.6x 26F threading. If you want to experiment with the bottom half of a modern headset, I have half of a cheap Ritchey that I'd be happy to send you.