You'd kind of expect Gibson to do a fin like that with a rosewood or ebony fretboard, but otoh it looks like something you could have certainly asked for in the seventies. After all my pelham blue Grabber has a black painted neck, back of headstock AND black painted maple fretboard too and came that way from the factory.
Whether legit (probably is) or not, the fin is lovingly done and gives the whole instrument a more valuable appearance. If it's a refin, it was done well. Very nice specimen no matter what the story behind it.
Uwe