Its a Jetsam micro booster I'm rebuilding (some parts being machined for me), I suspect the seal will be the same as in the baby booster, but I haven't stripped down my baby booster yet to check.
I found a couple of companies selling seals on the net but not been able to find the right size yet. Its a U-cup type design rod seal, with internal spring. Feels like PTFE or filled PTFE - not sure what the best material for high pressure O2 is, I suspect PTFE.
Size is (inches) 0.750 External diameter, 0.625 internal diameter (so cross-section 0.0625), and 0.125 thick. Most seals seem to have a square cross-section.
Any suggestions where to look welcomed.
I've emailed Jetsam as IF they are the same as the baby booster I should be able to buy from them - not heard back yet though.

Neil