This configuration was standard on the Peugeot UO-8/AO-8 until they went to stem shifters in 1974. It allowed the familiar UO-8 10-speed to share a frameset with a European 5-speed model and simplified manufacture and inventory control for the company. The single-boss configuration probably did start in the 1930s or 1940s with nonexistent or suicide front shifters, and it was easier to add an asymmetrical clamp than to add a left-side braze-on selectively.

When I converted my Peugeot to barcons, I had only one braze-on to knock off.