There were indeed Nortons with a cut off petrol tank. Basically the earlier Australian machines and one contract C9062 built in 1941.
For some strange reason when the tank top filter was made a standard part on the machines later in the war they did not use the cut off version petrol tank anymore whereas the other brands like BSA introduced them on their machines!
The petrol cap hinges on the left hand side of the opening, instead of the rear end side as used on "normal"tanks.