Choose which Soda Stream adapter from the drop down menu: Australian (CGA320) or USA/Europe (6ACME) for the type depending on where you are as the threads are different!
The spare o-ring available on the drop down is NOT the same as the included one. The included one is a hard pressed fibre seal (green) that is long lasting but will need a spanner to tighten. The spare is a EPM seal (black) and is softer, it will allow hand tightening however will become damaged more quickly.