It's silver plated. The mark looks like the "MERIDEN S P CO" trademark used by the Meriden Silver Plate Company. Beneath is "INTERNATIONAL S CO" for International Silver Company. The Meriden Silver Plate Company was based in Meriden, Connecticut and became a division of the International Silver Company in 1898. The half-circle trademark on your cup was reportedly first used in 1921, although it was not officially registered until 1923.
Update:
On closer examination, the upper portion of the mark looks as if it might read, "MERIDEN B CO" for the Meriden Britannia Company, also of Meriden, Connecticut. Britannia metal was a non-silver, white metal alloy; however, the same firm produced silverplate wares. Most of the Meriden Britannia Company marks that I've seen are circular, with balance scales on a shield in the center; but since their silverplate line was also acquired by the International Silver Company in 1898, maybe a "MERIDEN B CO" version of the same half-circle trademark was registered and used in the early 1920's as well.
