You have a Springfield Model 1884 "Trapdoor" rifle. 45-70 Gov't caliber. It has the Buffington rear sight - adjustable for the first time in both windage and elevation. Don't be surprised if the windage knob does not move the rear sight right or left. Seeing as this was the newest "gee-gaw", unit commanders could direct that their rifles be delivered with the windage feature 'pinned' so that it could not be used.
"No man's life, liberty, or property is safe, while Congress is in session." Mark Twain