Erskine Allin was the master armorer at Springfield, and he developed this conversion design based on utilizing the 1863 percussion rifles left over from the Civil War.
The conversion was designed to take advantage of the sizable quantity of muzzle loading Model 1863 Springfields that were stockpiled following the Civil War. The overall design and philosophy of the conversion (remove a section of the breech, mill a chamber into the barrel, install a hinged breech block) remained consistent through all produced versions of Allin's system.
The first models (left and above) retained the 58 caliber bore, a mechanically ingenious spring loaded extractor/ejector assembly; opening the breech extends a cam-driven extractor, while retracting an ejector pin mounted in the bottom of the cutout, and when the breech is fully opened the extractor snaps back and the pin strikes the extracted cartridge, ejecting it from the weapon.
The second model, 1865 (below), had a simplified and improved extractor and the barrels being relined and rifled to .50 caliber and chambered for the powerful centerfire .50-70 Government cartridge.