Man, I think you'd go crazy if you just left an atty stock.
I can take some pics next time I take it apart.The o-ring is on the very bottom of the head at the base. The smooth part of the base of the tank slides up and down on it when you tighten or loosen it.
Ian - Sorry, don't know where I was going with that either. I've been quite sick since then and just got out of the hospital yesterday after being in for three days. Haven't worked on mine and probably won't for a few days yet.
Thanks for the reminder Cy. I couldn't get a pic to come out right so clipped this one off the net. Place the o-ring so it touches the base.