I figured I would add to my previous post and list which o-ring size is used in which locations. That way if you don't have access to aqualung part numbers this can still help you. I know that different manufacturers give the same parts different names so I've given the names I would use (or that the aqualung catalog uses) and, in cases where I've heard other names, included a brief description so you can figure out what it is if you're used to a different name.
O-Ring size / Uses
First Stage:
011 / High pressure crown (what the high pressure seat presses against), Around brass filter, Low pressure port plugs/hoses
006 / Inside of balance chamber (AFTER you put in the backup ring)
005 Backup Ring / Inside of balance chamber (BEFORE you put in the o-ring)
013 / Outside of balance chamber
012 / High pressure port plugs/hoses
Second Stage:
015 / Valve body (regulator hose side)
010 / Crown (the metal part that screws into the hose side of the valve body), Swivel at end of regulator hose
001-1/2 / End of shuttle valve (the plastic part that moves when you breath)
007 / Outside of adjustment screw (small plastic screw that presses against back of balance chamber)
013 / Outside of cam (plastic piece that the adjustment screw screws into and the adjustment knob attaches to)
018 / Outside of valve body screw (the plastic piece that the cam fits into that is on the very end of the valve body on the side opposite the regulator hose)