This is the sight I put on my new Uberti Model 1876 Winchester in .50/95. It got me right to the 1,000 meter buffalo on our long range. It fits a 3/8" dovetail.

http://www.brownells.com/rifle-parts/sights/rear-sights/m-95-long-range-sight-prod18599.aspx

Now I need one for my Marlin, .45/70.