My long running theory is that the latch tray bracket somehow interfered with the jig or sequence that the jig was used; when the typed the numbers on the radiator support were more commonly placed.

This seems to work on several levels for Superbirds. 1st, it seems to be a rather occurrence on Superbirds. 2nd, it gives a specific reason why it is common on Superbirds. 3rd, since the latch tray support was somehow integrated on the assembly line process before painting the car, and since it was a very non-standard item as far as the common b-body assembly process occurred; some cars could have easily had the #'s installed before the support was spot welded on and others could have had the support installed before the #'s jig got to the car-preventing the jig from being able to latch on the car.

I expect credit for advancing this theory. No Nobel prize, but recognizable credit.