Marble's Tang Peep Sight for Winchester 1876, 1886. Marble's Model number 9801. Blue steel. I had it on a Uberti 1876 repro and it fit fine.  Machined steel rear sight is fully adjustable for windage and elevation, with each positive click equaling approximately 4/10" movement at 100 yards (varies with sight radius). Comes complete with three different apertures, 0.032", 0.055" and 0.102".

I can meet in the Mesa/AJ/Gilbert area most afternoons. I also get over to the west side every couple weeks. Not interested in anything other than cash offers. If the posting is up it's still available no need to ask.