PLAIN ENAMEL
The mid 1950's would see the invention and rise in popularity of the "dual coil" or "humbucking" style pickups.
Plain Enamel was used in a majority of these pickups and could be identified by its dark black color.
Because Plain Enamel magnet wire was typically manufactured with a thin coat of insulation, now known as "single build", more turns could be added to the coils to enhance pickup coil performance.