1974 Norton Commando bought in boxes all apart. Oil tank (stock as all Commando oil tanks, not after market) has 3 vent tubes on top of the tank. Rear one comes from oil fill opening as breather to crankcase. Middle one (same size as breather one) goes to the rear air filter plate to put fumes, etc. back into carb. Third one is at the front top of oil tank and is of much larger diameter then the other two tubes. This tube goes down forward about 3 inches. I have no idea what this extra tube does nor were it goes. Looks like factory made, not added on by somebody. Does this go to another year model of Commando or ? Thanks for the help. Don't know how to post pictures, sorry.