Ooouf. Trawling the usual French forums, and I'm not the first to raise the question. So far, the best answer is 1950.
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There seems to be an infinity of time though before 5 speed went down to amateur and everyday level, and during this time 3 and 4 speed were still very common. I'm suspecting by the time 5 speed blocks got to amateur level rear mechs had well moved on to parallelogram.
The main reason for the question is I'm messing around with this "new" vintage wheelset where I can't find much date information. I can't make head nor tail of the thinking behind putting a 4 speed block on a Exceltoo 120mm OLN rear hub which was most certainly designed and left the factory for 5 speed. To get everything "right" for 4 speed, I've had to swap spacers and cones around, reduce the dish, and then add a washer behind the 4 speed freewheel to make the freewheel sit closer to the drop-out. It's been a PITA but at least now the chain line is where I want it to be.