In addition to the above, there IS an o-ring which is internal to the vacuum breaker and is used as the face of the valve which seals to the nylon(?) or teflon(?) seat indicated as Item 3. See below pic:

The arrow for Item 1 is pointing towards the tiny o-ring whose OD is about 9/32" and whose ID is about 9/64" - measuring o-rings is
NOT my forte. The diameter of the brass shaft that the o-ring slips over is 0.150". Can you find one at the local hardware store that would work? Yes, but not for too long.
Item 2 is the valve housing which either screws into the boiler or a tee which is screwed into the boiler. The threads at the end of the arrow on Item 2 are the BSPP threads as previously described. They are sealed by means of a suitable sealant, teflon tape, or copper sealing ring (preferable).