It's a shame you cut the headstock off the full length photo so we could see it in relation to the whole bass. The other shot perspective makes the headstock look massive
That seller is a studio that appears to run a sideline business listing its instruments on Ebay for high prices. Take a look at its other items on Ebay.
I love to see the signs of wear on an old bass showing it has been played regularly through the years but sometimes they look like the previous owner(s) just couldn't care less for it. This one is borderline for me and some of the chipping around the body edge looks a bit "manufactured".
Obviously an original 51/52P in decent condition (with case?) is worth whatever someone is prepared to pay for it. If some serious (well off) Fender collectors were to bid against each other at auction who knows where it would end? If it's all original with case then it's a very rare thing; I don't think it can be priced accurately without knowing its true condition. Its provenance would be relevant too. Maybe a come buy me auction estimate of £20k+ wouldn't be unreasonable with the expectation it would go (much) higher.
It's genuine and definitely not pieced together any more than other Fenders of that period. All Fender guitars were made up from parts made at different times. The vintage year is traditionally determined by the youngest dated component such as the neck, body or pots. Fender would buy bulk pots and serial number plates and they would be picked from stock bins relatively randomly so there is a fair degree of date tolerance when it comes to serial numbers. IMO a genuine period case would've added at least £500, maybe a fair bit more, to the price. I don't think the strap buttons are significant as a reliced modern pair would be indistinguishable from the originals.
It's based on the original 1970s StingRays. Slab body, 2 band EQ, through body stringing bridge with mutes, 7.5" radius neck, aged look neck lacquer, vintage machine heads and battery plate.
It's a horrible rainy day here so I thought I'd do a comparison of my 63 (on the left) and 64 J's.
I cannot hear an output level difference between them. The 63 is wearing stainless vs nickel strings on the 64 which is evident in the slightly brighter tone of the 63.
The headstock of the 63 is 14mm thick against 15mm on the 64. Otherwise the neck profiles feel the same.
The body contouring of the 63 is cut very slightly deeper and feels relatively light weight for a J which is confirmed by my bathroom scales by approx half a pound at 9lb vs 9.4lbs for the 64.
If these were fitted with the same strings I doubt I could tell them apart blindfolded.
My guess is that the fretless fingerboard was supplied with excess wood at both ends to allow for whichever neck it was to be installed on. Unfortunately whoever fitted it didn't account for that and just slapped it down regardless, probably thinking "it's a fretless; who needs markers? Play by ear".
You would think that the intonation problem would be the same for all strings. If you measure the distance from the nut to the first fret position is it the same as on another bass (if you have one to compare)?
If that distance is a few mm more then maybe a zero fret is required?