That’s the one inside the tunnel, or cavity, that the regulator goes into. The one I jokingly said has put men on their knees crying, or I should add all their hair pulled out.
Since your block is anodized black, it’ll be hard to see. You need a really bright flashlight held at an angle. Where the threads inside end, that o ring is just under the threads.
Take your wife’s longest and sharpest sewing needle, bend the tip slightly, about a 1/8” bend, and “prick feel” for it. Once you get a good stab at it, pry it out with that needle.
Clean that hole cavity with a q-tip soaked in acetone to get all the old lubes out. Then take a shop compressor blow out tool and blow out the cavity with short bursts.
When installing that new 10 x 1.5 o ring install it dry and lube it in place with silicone grease on a long tooth pick.
If you pre lube it and try to install it wet, you’ll pull whatever hair you have left off, and put all your pcp stuff in the classifieds-
