Looking at the photo gallery on the sale site and comparing with some of the Thruxton at the National MotorCycle Museum in Birmingham (not the one in Alabama), I think the oil tank is either genuine or a really good copy. Every tank will have been fabricated individually but perhaps they made a batch. Stuff from the Norton race shop has distributed itself around the world, via the back of peoples' vans, so who knows. Well maybe Norman White knows 'cos he was there, you could ask whether he has/had some or made copies. Or suggest to Joe Sieffert (AN) that he should make some for sale now he has a pattern.
The battery on the Thruxton lives in the seat hump, same as JPNs. But it doesn't look like the Commando in the listing does the same. Perhaps it runs off the alternator.