Very nice weapon. Makes me miss the one I had in my youth. Shot some CCI blazer ammo out of it and one of the casings blew out about midway up and scored the heck out of the cylinder. Little did I know that you could replace the wheel for minimal amounts. Sold the gun and kinda regret doing it now.
It's a rear sight blade replacement from a company called One Ragger Hole. The one shown is the larger of the two sizes. The smaller is great for target shooting and the lager is great for hunting. They can be had from brownell's or for the company website. (which I don't have a link too.) Fairly cheap in price, works nice.