It's just catalog based knowledge, too, but i have a definitely 1973 French catalog showing the Sur-Mesures and the Tour de France model with a downtube clamp and clamp on housing guide, and what seems to be a 1978, with braze on shifter mounts.

But with the Moto line up in general they produced different models for so many different markets , even with the same names, plus their artisanal Sur-Mesures line, it is probably impossible to pin down an official date. Just like their seemingly flexible serial number policy, i bet it happened a bit piece-meal ... i mean it happened artisinally.