The first in the long and hugely successful series of Winchester lever-action rifles, developed from the 1860 Henry rifle.  The Winchester '66 can readily be distinguished from the Henry by the presence of the King's Patent Loading Gate on the right side of the receiver, and from subsequent Winchester lever guns by the fact that its receiver is, like the Henry's, made from yellow gunmetal (a bronze/brass alloy) rather than steel.
winchester model 1866