I have a bit more information for you. The total number produced for the US military was about 20,000. It was the last 5,000 that were chambered and configured for the 56-52 spencer cartridge, contracted for in March of 1865 and delivered a few months later (some of these last cartridge arms wound up in the French forces during the Franco Prussian War). Some of the units issued the Gallagher were the 3rd, 4th and 6th Ohio, the 13th Tennessee and the 3rd West Virginia. If you want some of the brass cartridges that it took, several reloading firms that deal in obsolete cartridges offer them but your arm appears to be in such good shape I would not shoot it (you could reduce its collector's value) but having a few empty cartridges near the carbine will look good. Judging by your high serial number I would guess sometime just before the contract called for the altered gun to be produced so I would guess late 1864, early 1865. The Gallagher was not considered a reliable firearm according to a survey of officers of units issued them because of extraction problems. The early cartridges were very thin and made of tin and with no extractor the fired cartridges often tore apart as they were digitally (as in fingers) extracted. I hope this helps.