I changed the ADV diaphragm on my unit tonight and had a look at the stem.

It doesn't look like the pictures I have seen of the first or second gen versions.

It appears to be a white nylon cone seal that seats inside the ADV bolt without any o ring, so very similar to the gen 2 but in a much earlier unit.

Did anyone else come across this version, it appears to be retty robust but I was fully expecting the version with the o-ring.