I would guess by looking at this they used formvar. There seems to be a pattern where Fender used formvar in all their pickups until somewhere around 1964 where plain enamel was used, at least in some pickup models.
It might not be a bad idea to confirm this with other internet search results. Maybe try 'vintage pickup specifications' as the search string.
I'd be surprised if they wound the bass pickups with a different wire from their other pickups, since they looked at it from a cost perspective and would have had no reason to favor one type for Strat pickups, and something else for their bass guitar pickups.
I suppose by looking at pictures of sold and for-sale eBay or Reverb listing for authentic vintage pickups, you might be able to corroborate 1964 as being a year where the color on the wire changes. Formvar can be lighter or darker shade of orange, but it appears to me that plain enamel is always a very dark burgundy, so it might be possible to verify through visuals of pickup listings.
Seems that 7ender used different wire for bass PU than guitar PU.
" 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. "