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I'm sorry, guys, my bad. It's not an Mk5, it's an Mk2. Brandnew.
The MK2 is considered an excellent reg for O2 use, so I imagine there's a good seat for O2 available. The seat is much smaller than the MK5 seat, and theoretically would provide less fuel in the event of a fire, but honestly I have no idea if that's even a consideration. It's just a simple plastic plug that fits in the end of the piston. One could easily be made.
There are only a couple of o-rings, no dynamic o-rings that are subjected to HP air, very simple air path, probably a very good choice for 50% O2.