Originally Posted by
SW84
I believe it attaches to the area below the derailleur that looks like a "C". I suspect it's a small metal tube that's held in place by a bolt. Does anyone have any ideas?
Looks like a housing stop. Some older bikes had top tube routing, and this was a way to make the top pull compatible with the bottom pull derailleur.
Trek used this setup in the early 90s. By this time, the derailleurs did not have the stop, so a braze on stop was used; same principle.