The Model 1865 First Model Allin Trapdoor design was invented by Erskine S. Allin, Master Armorer at Springfield Armory, as a solution to converting the massive surplus of, then outdated, muzzle loading rifled-muskets manufactured during the Civil War to a more modern breech loading design. Only 5,000 of these First Model Allins (also known as the Model 1865) were manufactured by altering existing rifle-muskets to the Trapdoor system in order to allow loading from the breech end with a .58 rimfire metallic cartridge. After only one year of production, Springfield shifted to manufacturing the simpler .50-70 centerfire Model 1866 Second Model Allin conversion rifles, making any surviving First Models rare and desirable. This example features an "1865" dated lock, "1864" (partial) dated barrel, boxed script "SWP" (Samuel W. Porter) and oval script "ESA" (Erskine S. Allin) inspection cartouches marked on the left stock flat, and "US" marked buttplate tang.