Gomiata:
I have a P.A. 20 ga with a serial number about 1,200 less than your prospect.
The shotgun was from my grandfather and I am very happy with it: it has an original unusual choke combination: imp cyl and full (0.003 and 0.025") and with a proper spreader shotshell in the left barrel I used it to hunt quails (both, bobwhites and a local species called bannded quail) with very good sucsess.
I think all the Pedro Arrizabalaga shotguns made in that era (60´s and 70´s) have a similar good quality regard the grade. They are hand made and very well finished; only drawback is the "naif style" engraving for the animal scenes.
Now, $10,000 USD is a lot of money and I am not sure if a P.A. of that era worth that summ.
Best regards,
Jose