After about a 5 second search with Google, here's a brief description of Mr. Whitmore;
"WHITMORE, Nathaniel .—G Rifle maker. Born at Mansfield, Mass., in 1829, and learned the trade in his father's shop. Worked for Sharps Rifle Co. and Remingtons. Died at Eastondale, Mass., in 1917. Maker of a heavy barrel, muzzle loading, percussion, target rifle with double set triggers. A presentation rifle by Whitmore was gift to Gen. U. S. Grant from the citizens of Providence, R. I., and is now in the Smithsonian Institution in Washington, D. C."