The origin of this probably dates back to lever front derailluers, an adaptation bit to bring cable shifting to existing bikes. While it is oddball, it does keep the clamp from sliding down which happens to cheaper clamps and or those without a brazed on bit to stop the slide. If you must braze on a bit there, why not make it useful?
Huret and Simplex made clamps to work with this.

I have a Bertin frameset that has this unit, a C-34, nice frame, rides beyond its Durifort tubing. The clamps surface on ebay from time to time, often with descriptions that only guess as to their purpose.

Very French. How can 19 million Frenchmen be wrong went the Huret advert copy, or something very close to that...