Designed by Erskine S. Allin, Master Armorer at Springfield Armory, to correct multiple problems encountered with the Model 1865 conversion rifles. Approximately 25,000 Civil War era muzzle loading rifle-muskets were converted at Springfield Armory to Allin's Model 1866 breech loading system and rechambered to .50-70 CF. The Model 1866 received it's baptism by fire in the hands of U.S. troops in the Hayfield Fight and Wagon Box Fight, both a day apart on the 1st and 2nd of August 1867 along the Bozeman Trail, in which the severely outnumbered U.S. troops successfully held off several hundred Native American warriors. This example is dated "1864" on the lock and "1866" on the breech block. The left stock flat is stamped with a boxed script "HSH" inspection cartouche.