Looking at the design and more importantly, the fit of that seat I question whether it was originally made for a commando. It could be an old manx or other brand fitting.
The latter style norvil seat/tail sections were more square and a bit taller. The yellow one has the number plate incorporated whereas the 1st one doesn't. Try to do a search or Norvil Production Racers and see if you can find picts in online brochures from the past etc.
Buying the tank and seat together will ensure bolt on compatibility. Buying separately may require some custom fitting on your part.
I have a Norvil proddie tank & seat.
On the original paper work it tells you how to!
"welt over the top seam of the oil tank"
The rear of the petrol tank sits on it!
A little bit of fettling to suit! I think so Sir!
all the best Chris