From the manufacturer:
For the first few years of their production, Gibson’s famous “Patent Applied For” humbuckers were wound using imprecise machines, resulting in pickups with slightly different output and tone. They were – and remain today – as very desirable to players who want to mix and match and explore a complete spectrum of tonal possibilities. The ’57 Classic vintage style humbucker is the result of Gibson’s drive to capture and recreate this renowned characteristic. Introduced in 1990, the ’57 Classic provides warm, full tone with a balanced response, packing that classic Gibson PAF humbucker crunch. They are made to the exact same specs as the original PAFs, including Alnico II magnets, nickel-plated pole pieces, nickel slugs, maple spacers, and vintage-style, two-conductor braided wiring. The pickups are also wax potted to remove all internal air space and any chance of microphonic feedback.