I don't think that is nitrocellulose. Not sure what it is, but Fender had discoloration issues with its finishes throughout the early to mid 70's. Maybe an acrylic.
Matching only the worn spots is the kind of job to seriously consider turning down. It would be a problem even on an old nitro lacquer fretboard, because of the grunge on the bare wood spots, and the color matching. The usual on maple is to replace the frets and refinish the whole board.
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Howard Klepper
http://www.klepperguitars.comWhen all else fails, clean the shop.