I bought a frame side ball stud from Brewers which is for up to '76 A-bodies, but it doesn't have the flats on the threaded shaft to keep it from rotating when installed. The original ball stud I removed from a '76 Duster has the flats, and F-bodies also use the same ball stud.

When did the frame side ball stud shaft change in A-bodies?

Here are some pics of the two ball studs:





...and a detail shot of the old stud, showing the flats: