I'll add my Duster to the data points. Build date of 11/22/73, shock mount bumpers front and rear, all three holes but no threads on the bracket holes.
So it still goes along with most of the other stuff I’ve heard, since it still had shock mount bumpers. But I haven’t heard of one that had all
3 mounting locations that had a front shock mount bumper. But not threaded! So like they had started the frame rail modifications but hadn’t run out of shock mount bumpers yet.
Not that I’m any expert on it, but it’s definitely interesting. I’m just happy it went that way, would have been harder to swap on my ‘71 front clip if my car hadn’t had all 3 mounting locations.