While it is true the 2nd Model Allin conversions would have been issued to frontline troops, there were exceptions. For example, by "Sandy" Forsyth's own account, his scouts had several .50 caliber Springfield rifles at the Beecher Island battle in Sept. 1868. These most likely would have been the Second Model Allin conversions. While the later M1868 were probably produced in limited numbers in that year, the majority of M1868 were produced in 1869 and some in 1870, and were so-marked...as opposed to the true M1870. My photostated copies of the Records of Ordnance and Ordnance Stores in the Hands of the Troops, only go back to March 1870, and I don't currently have access to a film reader where I can go back earlier than that. The 1869 cartridge conversions of the M1859 and M1863 percussion Sharps carbines were in the hands of cavalry units by 1870, but probably not before late 1869. (Ref. the two books by my late acquaintance Frank Sellers.) It might be possible for a civilian to have ordered a cartridge Sharps directly from the factory, if he could afford one.
As far as any civilian obtaining the 2nd Model Allin from the military is concerned, there were some limited instances of the Army supplying guns to "citizens" (as was the term for civilians back then) to protect themselves from Indian attacks. The other possibility would be if the civilian was an employee of the Army Quartermaster Dept., as a scout, teamster, packer, etc. If, having left employment with the QM Dept., he might have either purchased it outright, or "lost" it, and had its cost deducted from his final pay. In 1876, the value of the then obsolescent Sharps carbine was about $15.56.
Hope this helps with your plotline.