When Gibson first began making humbucking pickups, the patent had not yet been granted for the technology, so the pickups featured stickers that said “Patent Applied For.” However once the patent was granted (in July 1959) the company shifted to using stickers on the pickups that featured the “Patent No 2,737,842″ label. (The change in stickers took a while — it actually occurred in late 1961 or early 1962.) Eventually, in the mid ’70s, Gibson began stamping the patent number onto the pickups, and eliminated the use of stickers.
The early patent number pickups were identical to the last generation of PAF pickups, though changes began creeping in as early as late 1962 (change to black jumper wires on the end of the coils and white jumper wires at the beginning of the coils). In late 1963, the magnet wire was changed from plain enamel to polyurethane coated.
In later years (around 1965 to the 1970s), the pickups changed to “T-tops” — which had a “T” embossed on the bobbin and chrome covers (instead of nickel).
Trivia: the original patent number that Gibson printed on the stickers (Patent No 2,737,842) was incorrect; it was actually a patent for a Gibson trapeze tailpiece. The correct patent number was 2,896,491. The debate rages over whether this was accidental, or was intended to mess with competitors who were trying to copy the humbuckers.