I would guess from the location just behind the BB, that this is the remains of a pin to hold the chain stay in place during brazing. It could be that a bit of welding rod was used as the "nail" a fairly common practice. They are often left undisturbed on the inside while the outside is filed smooth. I suppose it could also be remains of the much maligned, miniture, missing metal worms, as the photograph reminds me of a scene from "Aliens."
Sincerely ;-)>
Richard Long (RIX)
Temecula, CA
USA