The photo below shows the o-ring location after some air-pak disassembly has been done. The o-ring sits on a post in the air-pak and is not a field serviceable item by the members.
The photo below indicates the tapered area of the probe which is there to assist in the alignment of the post o-ring and the probe, and also the actual straight cut o-ring seating area. This seating area is where we identified some substantial scoring and galling on the units that had repeatedly failed.
The last photo shows another area that can receive some damage during day to day operations.
If the bottle is forcefully held out of alignment during installation of the bottle into the air-pak, a jagged lip on the internal edge of this face can possibly contact and damage the post o-ring. So try and be reasonably aligned before slamming a bottle home into the air-pak.