Mid 70's had exactly that same fracture at the ball.. but sorry, never actually got to the structure of the fault... They must be as you say threaded.. Will say though that it hung on for at least a year or two before I bought a replacement.. Matter of fact, I sawed off the ball and "bent/angled" the post in a vice and by coincidence it is a perfect hex fit for the mount socket of a Boss vocal monitor.. Still in use on occasion...
Cheers
John