I found this.
" Fender used 42 gauge (43 gauge in some cases such as teles) formvar for their normal production guitars until March ‘64, then they switched to 42 gauge plain enamel. However, Fender used plain enamel for the P-Bass after switching from the single coil pickup to the split coil pickup at ‘58 ❮should be '57❯. As a matter of fact, they used plain enamel for all their bass models until they went back to formvar in the late 60s. In the early 70s they returned to using plain enamel again. "
According to that guitars use formvar '50 to '64. After that PE wire. Basses: formvar '51 to '57 or '58, then plain enamel to late 60s. Then early 70s again PE.