Nice photo of a beautifully stamped 'acceptance cartouche'. "1900" acceptance date is nicely consistent with the likely assembly date, I listed above.

"JSA" is for Joseph Sumner Adams, assistant foreman, Springfield Armory assembly room. "Sumner" was from Maine and worked at S.A. from the 1860's, till about 1920.
His cartouche appears on U.S. Krags, after the death of master armorer S.W. Porter, in 1894.