Not all snap-in cage-type nuts were machine thread.
I know. I've used both types many times. The question was why the square hole in that fender location and was there a special clip or nut used.
Most of the Tinnerman or speed nuts we are familiar with today just clip over a straight edge or are just a flat rectangle which lays over an opening. Usually there is a simple hole in the metal so the screw can go thru that hole and into the nut for maximum support. Those wouldn't require a square hole and a smaller round hole would probably be easier to punch or drill. In fact, the square hole makes installation more difficult for the clip on type. It is also a bit weaker when using the clip on type because one side of the nut is not well supported.
Randy has a good question. Did Packard use something not found as easily today or just use the flat type and hold them in position over the square by tape or some other method while the screw was inserted? That was the method they used on some dash speed nuts where access to the nuts was restricted
Tinnerman nuts came in all manner of fittings/mountings, not just the slide-on type. Many had spring backs with stop prongs on them, which allowed a push-in-and-lock rather than slide-on mounting.
Of course the phrase, "Tinnerman nuts today" takes it into a whole new realm. Not many would use Tinnermans like the ones Packard used in such an application today since it is far easier and cheaper (both from engineering and manufacturing standpoints) to simply fill the hole with a push-in-place square blob of Nylon or some kind of plastic with expansion ridges that lock it in. Then screw into that with a sheetmetal screw. My late-model Cadillac has both Tinnermans and the plastic blobs. Chances are pretty good that if you check the rear license plate mounting on whatever late model car you may be driving, you'll discover the plate is anchored in plastic with a sheetmetal screw thread. Same kinda deal.
As for parts documentation in parts books... when it gets down to small fittings like these, the books can sometimes be misleading, if not just plain wrong. I can tell you firsthand that these books are written very early, just after initial body engineering is in place. But by the time a vehicle hits the assembly line, things sometimes change. These changes may be listed in service bulletins if management deems them important enough to acknowledge. Otherwise not. Issuing service bulletins costs money and takes time. And remember... the cathedral tail lights were engineered very late in the process. Thus parts documentation and fastener specs for headlight doors versus tail light housings were unlikely to be completed at the same times.
