43 AWG Heavy Formvar
FORMVAR
From the early 1950's through mid-1960's, Formvar was used by the era's foremost guitar manufacturers in a majority of their "single coil" style pickups.
The natural color of Formvar insulation is amber.
Those who use Formvar in their pickups today say that it produces similar tonal quality to those vintage pickups of the 1950's and 1960's.