Originally Posted by
HTupolev
I'm not sure that that can be well-characterized as an "origin." The Cambio Corsa was a rare example of a derailleur that
required a horizontal dropout, but it's not where horizontal dropouts started. For early bicycles, horizontal fixtures for the rear wheel were the norm, since they're the most minimal way to provide the ability to tension a chain. Track ends were initially more common, but by the 1930s, "modern" horizontal dropouts were making their mark; this meant that they were being used with all kinds of drivetrains, including derailleurs. For instance,
here's a 1934 advertisement for Cyclo, showing their Witmy derailleur being used on a bicycle with horizontal dropouts.
In the 1930s, derailleurs were the exception, not the rule. Even flip-flop hubs were an expensive upgrade. But even with a normal hub, horizontal dropouts accommodated whatever gearing a user wanted.
Post WW2, it was an interesting time. Campagnolo dominated the Italian market, but Simplex made inroads in Italy by using Coppi. I don't think there was much cambio corsa outside of Italy & and maybe some Swiss builders. And some French builders, at least Herse, were using vertical dropouts soon after WW2. When the Gran Sport was the derailleur in 53, Campagnolo ruled the roost. Whatever he wanted to make was then the standard. But from 45-53 (arguably 51 when the GS first came out but didn't dominate until 53), anything could have happened. It was an interesting period in bike history.