Found this from a old thread from Dino.
Here are the different O-rings
For a tight fit. Outside
90025K143
For a loose fit. Outside
9452K58
For the inside of the cap
9262K126
For the larger tank. Tight fit
90025K147
For the larger tank. Loose fit
5018T126
Alternative O-ring for inside the caps
5233T35
These are all McMaster Carr Part Numbers
McMaster-Carr
Here are the different O-rings
For a tight fit. Outside
90025K143
For a loose fit. Outside
9452K58
For the inside of the cap
9262K126
For the larger tank. Tight fit
90025K147
For the larger tank. Loose fit
5018T126
Alternative O-ring for inside the caps
5233T35
These are all McMaster Carr Part Numbers
McMaster-Carr
that!