It's argued that Ray Butts created the Filtertron humbucker for Gretsch before Seth Lover created the PAF humbucker for Gibson, making the Filtertron technically the first guitar humbucker in existence. Gibson managed to get their design in the patent office first though.
Despite the Filtertron being electronically similar to a Gibson PAF humbucker, they aren't constructed, or sound, remotely similar. The main differences are the double sets of screw poles, a huge 1/4 inch thick alnico 5 magnet, tall and narrow coils, and a very low wind count.
Filtertrons are usually made in different dimensions to normal humbucker. They are narrower, making fitting a Filtertron to a guitar that takes normal humbuckers impossible.
These humbucker sized Filtertron pickups are designed to bring the Gretsch Filtertron sound to any typical humbucker equipped guitar. Utilising the narrow coils, thick alnico 5 magnet, low wind counts and double screw coils, these pickups have the famously iconic rockability sound in a different size.
A Filtertron has a bright, cutting and jangly characteristic. This makes them a perfect partner for a naturally dark amp. Filtertrons are commonly used into a clean amp for Jazz, or into a naturally breaking up amp for low gain rock. Their superior articulation and openness to a typical humbucker gives them a distinctive voice that cannot be replicated by any other pickup.