http://www.rockislandauction.com/viewitem/aid/66/lid/3113

As the US Civil War drew to a close, it was quite apparent to everyone that muzzleloading rifles were obsolete, and any military force wishing to remain relevant would need to adopt cartridge-firing weapons. However, the Union arsenals had a million or more muzzleloading rifled muskets still on hand. How to modernize the weaponry without simply throwing away all those existing guns?

The task was put to Springfield Arsenal master armorer Erskine Allin in 1865, and he devised a "trapdoor" style of conversion to turn an old Springfield muzzleloader into a breechloader. He had the benefit of having seen the previous year's extensive trials of breechloaders, but the final product was his combination of what he judged to be the best elements of the ideas available.

The result has long been known colloquially as the Trapdoor Springfield, and this 1865 model was the very first of them. It was still in .58 caliber, and used a rather complex extractor system. It would soon be revised to make improvements to it, and ultimately a .45-70 model became standard for US infantry and cavalry forces - and remained their standard until the adoption of the Krag-Jorgensen in the 1890s.