Yesterday my 1984 Centurion LeMans lug cracked in almost the same location. The lug itself split apart completely, not the brazed joint, about 1/4-1/2" below the top edge of the lug where the 'paint' crack is on your frame.
Since posting about it other folks noted their frames cracked in the same place.
Is this joint a common one to crack on frames or if not all frames, Centurions?
Al