I don't recall if I've ever seen a discussion breaking down proper timing for hollow versus T shaped mast jacks. I wonder if it might be a situation where they used 2 different suppliers and both were used concurrently? Another jack related item I've never even seen discussed is that some components are occasionally seen painted dark green instead of gray. I've seen the hook and maybe the base painted green on some '71s. Including my friends June built '71 Challenger convertible that he has owned since '73 so very likely the original jack.
The slotted Bumpers came out in June of 1971 on a Challenger. The slotted Jack I believe is a 1972 use. Barracudas did not come out at the same time but I do not know a date for this. My Challenger has front and rear slotted bumpers and is an early June built car. It some times is hard to come up with a date because Challengers were running over a week late at this time so a Challenger with a example May 24th SPD would of been built in June but a Barracuda with the same SPD May 24th was built in May..