The serial number on the one I had briefly was on the back of the action IIRC correctly, it was number 6 I think.
Despite being described as good number 6 was actually a dog and potentially dangerous to use. That particular rifle was the only time I have had a bad experience trading on here and I am £250 out of pocket as a result!
Despite their rarity I am not sure how valuable they would be, but I would guess £250-£300 for one in good condition and safe working order.