Just a guess, but probably either a New Army or New Navy in .38 Long Colt. That was the standard issue U.S. Military handgun through the Spanish-American War and Philippine Insurrection and was replaced with the Colt Model 1909 New Service in .45 Long Colt and subsequently the M1911 in .45 ACP due to dissatisfaction with the performance of the .38 Long Colt Cartridge against Moro Tribesmen in the Philippines.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar