Earlwb,
It looks like you have a Marble's rear sight on your rifle. If you hold the top thimble while turning the bottom one clockwise it will loosen it (move it further down the shaft). Then the top thimble can be turned to push the peep sight up or down. When done, turn the bottom thimble counter-clockwise to bring it up against the top one to lock it in place. There's not a lot of space to do this; just a turn or less from "unlock" to "lock."
The one I have always seems to "self-tighten," too. Some Kroil and leather-padded small pliers (and a careful, light touch) are often necessary. I've seen few used ones that don't have the knurling flattened or otherwise marred, so a lot of people must have had trouble with the things.
I have one on a rifle I'm always trying new loads on, so I just leave the bottom thimble loose. Why Marble thought they needed a lock ring, I have no idea. The Lymans do fine without one, and I never notice the Marble sight moving up or down by itself, either.
The book on the 8 and the 81 has instructions on how to make the spanner that takes the end nut off the barrel jacket.