This trapdoor design was invented by Erskine Allin, master armorer at Springfield Armory, as a solution to converting the massive surplus of, then outdated, rifled-muskets manufactured during the Civil War to a more modern design. Only 5,000 of these First Model Allins (also known as the Model 1865) were manufactured before production shifted to the simpler, centerfire Model 1866 rifles, making any surviving First Models rare and desirable. It retains its Civil War era markings such as standard Springfield markings on the "1865" dated lock, "U" marked flat barrel bands, "US" marked buttplate, and Springfield "eagle head" proof on the barrel. The rifle standard Model 1863 features. The trigger guard is installed backwards. Includes an oil-quenched, "US/S" marked bayonet and leather sheath with frog.

Very good bright metal, flashes of original case colors, and 40% of the original oil-quenched black on the conversion block, with the balance a smooth grey patina. The sanded re-oiled wood is good with numerous minor dings and dents from use. Mechanically fine. The bayonet is very fine retaining 80% original oil quenched finish with the balance a grey patina, and the sheath is fine with moderate wear.