1) why would USA be stamped on the back of an MIM neck.
2) If Fender went to the trouble to put the little "Fender" decal on the back, why couldn't the put one on the front.
3) at minimum it looks like the pickups and knobs have been changed on the Jazz.
Here are pics of my VM Fretless for comparison.
If you are serious about them, offer him $400 for the pair. He may be undervaluing the P bass while over valuing the jazz. Hell, he may be being completely honest and the guy he got them from fed him a load of bull about the jazz at the time.