As a fan of these gtrs I have been curious about the pickups for some time. In the late 50s some of them have the patent number 2683388 on them, but only for a short time (in 1958?), like this:
I thought maybe this was a new patent issued at the time, but have just looked it up, and it was submitted in 1952, granted in 1954:
This is interesting (to me at least) as people seem to assume the cosmetics are meant to look humbucker-like, even to fool people into thinking they were humbuckers, but Seth Lover didn't submit his hummer patent until '55 - in fact the Keller patent is cited as a reference on the humbucker - because of the screw-pole pieces, I'd guess.
Why did they put the patent number on the pickups in 1958? Dunno. Any theories welcome. Maybe the PAF humbuckers were causing a stir? There's a story there somewhere. Lover's patent was granted in '59.
EDIT - checking the full patent document - in fig 3 the magnets are numbered 17, so that the magnetised pole piece screws form the opposite side of a horse shoe magnet-type arrangement with the base, numbered 18 (referred to as the pole piece on the document).