US Pat. 4,220,069 covers both a hum-bucking design as well as single-coil. The Magnetic Field Design (MFD) design as depicted on the front page as well as Fig. 6 of the patent shows a copper U-channel encasing both the ceramic bar magnet and the windings of the single-coil with pole pieces. However, such a configuration is only found on the F-150 prototype. The single-coil MFDs on the S-500, the first production model featuring such pickups, use just a simple (steel?) baseplate as seen below in a beautiful 1983 G&L exploded view slick. Conventional single-coil pickups have magnetic slug pole pieces. The MFD design however uses soft iron pole pieces to channel the field of the bar magnet underneath to the strings for a more even response over a wider frequency range. This post by Gabe Dellevigne explains some of the idiosyncrasies of that pickup. Before 1988, its covers had square corners making guitars from that era easily recognizable. Later, the covers got rounded corners as on this S-500 Signature, although one could order the S-500 with old specs (see this 1988 “late first style” S-500) until late-1991. Beyond the slight different location on the guitar in the first place, the pickups on the Nighthawk, introduced in 1983, also differed in several other aspects. Whereas the S-500 can sound bright, the Nighthawk has a mellower sound due to a slightly stronger ceramic bar magnet combined with less windings, i.e. lower DC-R values. A similar approach was used to the differentiate the F-100 vs. G-200 MFD humbuckers. Shortly after introduction the Nighthawk was renamed to Skyhawk. Hence the name for these pickups which also found their way to the ASAT III. This BBE-era ASAT III has contemporary (i.e. BBE-era) S-500 pickups, for the pre-BBE version the pickup DC-R values need to be measured to assess whether the S-500 or the Skyhawk variety are present on the guitar. This pre-BBE ASAT III w/maple board has the more commonly used S-500 pickups, which also seems to be true for the middle and neck pickup on both this ASAT III HSS w/DFV and ASAT III Signature HSS, whereas those on this pre-BBE ASAT III w/rosewood board are more likely Skyhawk pups. The same Skyhawk pickups were also used on the SC-3 within the ‘entry level’ models. Even more, in conjunction with the introduction of the 1st style SC-3 in 1983, one could finally buy a drop-in replacement set of these pickups. The reading for the DC-R values of these pickups is pretty consistent across the board around 4.5-4.9kΩ for S-500 pups and 4.2-4.6kΩ for Skyhawk pups, with the high value associated with the bridge pickup. The exceptions to the rule are the values for this early Skyhawk and 2nd style SC-3 w/ebony board, which are both more in the 3.9-4.1kΩ range. After having been connected straight to the body, with the introduction of pickguard equipped versions, like the 3rd style SC-3 and SC-3 Deluxe, the pickups now were mounted on the same thing as the other controls. At the end of Leo’s lifetime, and just after, some interesting combinations involving S-500 pickups were devised. Several instruments, I know about an Interceptor and this Superhawk prototype, have 2 S-500 single-coils combined into a humbucker. Within a year after Leo’s passing, an employee created the ASAT Signature ‘500’, with an S-500 pickup in the middle. About 1½ decades later, around 2007, Alley Cat Music in State College, PA created the ASAT™Cat guitars based on the same idea. By then, the S-500 pickup used poly coated wire, instead of enamel, and many more windings to be much hotter and DC-R values typically in the 4.75-5.05kΩ range. This is true for this 1996 BBE-era ASAT III as well as for the S-500 pickups on the first Guitars by Leo community developed guitar, the ‘www.guitarsbyleo.com’ Limited Edition from 1999. The middle pickup of the ASAT Classic ‘S’ is also an S-500 pickup. The picture below shows the clear difference between the design of this pickup compared to an ASAT Classic MFD pickup.