Here's some info.
Vintage Guitars Info's
Gibson PAF (Patent Applied For) Humbucker Pickup Info.
I guess we should start with a little history of the Gibson PAF pickup. By the mid-1950s, Gibson wanted to counter the latest electric guitars introduced by Fender. Leo Fender had built a company that was a sizable competitor in the solid-body guitar market place. Gibson believed they could beat Fender with their high quality Les Paul, and by developing a low-noise pickup.
The problem with Gibson's P-90 and Fender's single-coil pickups was inherent in their designs, allowing 60-cycle hum (noise) to interfer with the sound. Seth Lover was the Gibson engineer assigned to solve the problem. Seth connected two single coil pickups in series (opposed to parallel) and connected the coils out-of-phase electrically and magnetically. Thus the signal noise of each separate coil canceled out the noise of the other coil. That is how the pickup came to be known as a "humbucker".
Seth/Gibson filed their patent for the pickup design on June 22, 1955. Gibson added the new pickups to steel guitars in 1956, and in 1957 on electric solid-body and arch-top guitars including the Les Paul Model. During late 1957, a small black decal with gold lettering was added to the underside of the pickup that read, "PATENT APPLIED FOR" (hence the PAF abbreviation).
Seth Lover received his pickup patent #2,896,491 on July 28, 1959. By mid to late 1962, Gibson changed the pickup decal to read, "PATENT NO 2,737,842". Interestingly the patent number listed on the decal was not for Seth's pickup design but was for Les Paul's trapeze tailpiece! Perhaps this was a research roadblock for the competition, or maybe just a mistake?