JS fine art is in Banbury, I'd guess that Joe, the owner, has either not noticed the serial number issue (which an auctioneer should pick up on really) or, as it's almost certainly been removed a generation or two ago, they felt of no matter (it ought to have been in the sale notes though and wasn't).
Having now removed the sight you wanted, I'd just keep it for plinking around the garden with knowing it getting knocked about wouldn't matter.
That or sell it on as you said was you're original thought.
Which I assume you've have decided to do as you have it listed on free ads now, and if you obtain your asking price versus the hammer price you get you sight for nothing and some change.