Looks like a new fingerboard to me. The neck isn't sitting in the pocket correctly either - should be an easy fix, loosen the neck bolts and straighten the neck in the pocket.
But yea, the neck itself is original, the FB has been lifted or cleaned up & refretted & re-dotted, or its a new FB.
I'd want to play it first if trading with a vintage Fender, although it's a move I'd definitely make - I'm not overly familiar with the Mustang values, but from what I've seen on the second hand market that seems like a great deal? The FB looks to have been done really well, so if anything this is possible a good thing as it's probably been any number of bad things repaired! Checking it out in person will be the only way to tell of course