don't know, but I suspect the late 40s/early 50s and probably a lot had to do with Campagnolo patenting the 5mm socket crankbolt dustcap design and the 6mm derailleur pivot bolts on the Gran Sport.. i've wondered too if this is right when the T-wrench was first introduced..
also, like you said, there are the ambrosio/3tt stems with 7mm hex socket, also found on the earliest versions of Cinelli 1/a from a similar period
andrei padlowski ny, ny \
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When did allen bolts become popular on bicycles? Especially with stems. Some of the early 3ttt used them, but just curious when they first stated showing up in normal use.
Sean Flores
San Francisco, CA