I don't know who did the water bottle cage braze-ons first. René Herse used them in 1948 - maybe even earlier. He modified Vito bottle cages (brazed in some washers?) to make them work. (Not entirely sure on this, he may have brazed his own cages that looked like modified Vitos.) In the 1950s or 1960s, Vito offered a different braze-on mount cage. Some other French "constructeurs," e.g., Alex Singer and Jo Routens, did not use that feature as early, even though their frames featured all other braze-ons (Pump pegs, brake and derailleur cable stops, brake bosses, etc.)
The reason for the braze-ons was that the French cyclotouring association decreed that a touring bike should not feature clamps that can loosen and slide. I guess most took this less literal than Herse, who decided it had to apply to bottle cages as well.
Jan Heine, Seattle