So far I found this looking at http://oldguns.net/cgi-bin/f2f/f2f.pl?http://www.oldguns.net/q&a11_00.htm That seems to match your description.
Maker Model Caliber Barrel Length Finish Serial Number
Winchester 1876 Maybe 45-75 ( Not Marked) Unsure Blue 84XX
Winchester's- Repeating Arms, New Haven. CT. King's Improvement-Patented-March 29, 1866. October 16, 1860 (Stamped on top of the octagon barrel in front of the rear sight). Underside of the stock to the rear of the lever lock believed to be serial number 8421.Top of stock located to the rear of the hammer " Model 1876". This rifle was carried by my Great Grandfather during the settling of the Oklahoma territory, or so I've been told. I'd appreciate any information you could give me concerning the history, value etc. The gun is at least in "very good" condition. I am also seeking information concerning ammunition. I am an avid hunter and I'd like to be able to take a deer with a rifle that my Great Grandfather may have taken buffalo with. Can you recommend a specific book that might shed some more light on this ? Thanks for your time. Mark
Mark- Sounds like a neat old gun. Made in 1879, collectors call your the "late second model." The caliber is usually marked on the barrel near the receiver, and on the bottom of the brass elevator block that raises the cartridge inside the action. Once you figure out the caliber (.40-60, .45-60, .45-75 and .50-95 are your basic choices) you can start looking for ammo, which will be hard to find, and probably a tad expensive. A Winchester "Factor Letter" from the Cody Firearms Museum would probably tell when and where it was initially shipped, but tracing it after that is nearly impossible. The value in NRA antique good condition for the Model 1876 according to Flayderman's Guide is either $650, or $1,000 or $2,750, and in fine, the same models are worth $2,000, $3,000 and $6,000 respectively. We cannot guess which of these values applies to yours because you didn't provide the barrel length to help identify it as the carbine, rifle, or musket model. Now you know why we take the time to specifically ask for barrel length. It is not a scheme to increase ruler sales, but a way to help collectors get accurate information. Book recommendations: "Flayderman's Guide" ($21.50 plus $4 shipping- see our book page) for basic information. George Madis "The Winchester Book" for lots of minute details and photos on all the older Winchester models. There is a brand new book out (title???) specifically on the 1876 Winchester that would be fascinating to the owner of the unbelievable display of about TWENTY 1876s in .50 caliber we saw over in Denver last year, but may be a bit more than many of us really need. Hope this helps... John