Developed by Erskine Allin, Springfield Master Armorer, the Allin Conversion was designed to take advantage of the sizeable quantity of Model 1863s 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 breechblock) remained consistent through all versions produced of Allin's system, but the First Model has some special features, such as the retention of a 58 caliber bore, a more sweeping profile to the top of the breechblock, and a 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. Though an elegant and mechanically ingenious solution for rapidly clearing spent cartridges, the ejector/extractor was eliminated from later models in favor of less intricate extraction system. Blade front and proper 2-leaf rear sights with "64" marked ahead of the tang and an eagle head on the upper left flat of the barrel. The lock plate is marked "1865" behind the hammer and "American eagle" and "U.S./SPRINGFIELD" ahead with the checkered breech lever resting in the bolster cutout. Smooth straight wrist stock with three "U" and "V" marked barrel bands, tulip-head ramrod, "ESA" (Erskine S. Allin) and "SWP" inspection cartouches on the left side flat, steel trigger guard and smooth "US" buttplate.

Very good as refurbished. Some light spotting and handling marks on the gently cleaned barrel, bands and trigger guard, with a dark brown patina on the tang and minor pitting near the breech and muzzle. The breechblock assembly shows a mixed brown and gray patina with darker traces of the casehardened pattern on the hammer and lock. The stock is refurbished and repaired with some cracking near the breech assembly and tang, mild chipping at the ramrod channel and buttplate, and scattered pressure dents. Mechanically excellent, including the ejector and extractor. Like all weapons that undergo scrutiny for military acquisition, the First Model Allins were subjected to a battery of tests and trials that can be fairly described as brutal, and survivors are rare in any condition.