This petcock will only work with tanks that have female threads. That means the threads on the tank will be on the inside of the opening.
2 red gaskets are included so that you can get a better angle on the shut off valve. With one gasket you’re stuck with the valve at whatever position it’s in when tightened to the tank. With this one, if you don’t like where it is after tightening it up with one gasket just pop in the second one and it will align in a different position.