Jim.
Are those fenders showing signs of dipping?

This is what may be at hand with these strange artifacts.
I presume that an early run of the fenders, doors, hoods and scoops etc, both Dodge and or Plymouth were indeed .018 or so. They probably proved to be problematic at speed and were then adjusted later in the run to be stamped out of .024 or so.
I say this because a reliable source with three NOS A-990 fenders in his possession from both Plymouth and Dodge came up with a mic'ed .023-025 reading and he said that they showed signs of dipping. (a sandy dark finish)
Mine in contrast are super smooth clean metal with no signs of dipping and are reading .024-26 or so where the mic can reach.
This is indeed a mystery and probably another case of the old Chrysler Corporation not keeping good reliable documentation of their updated and or changed memos.

Maybe so, and I stress this as a possibility that maybe Chrysler stamped all the fenders out of .024-26 thickness and then decided to either dip them further for removal of steel or suggested to their racers to do so. Remember, that even though most 65' Altered wheel base AFX 10/15 percent cars main bodies were wholeheartedly dipped in California, supposedly, Dick Landy decided against the practice and his car survived better than most as a non dipped main body car.

I just wonder it strange if fenders that show signs of dipping could be reading in the same ball park thickness as mine in virgin metal.

Do you have the doors and hood?