The USA mark on the base of a piece of vintage pottery identifies it as being American made only. Such pieces were marked accordingly by a myriad of potteries in the United States during the first half of the 20th Century.
The most popular pieces are from the McCoy Pottery Co. (Roseville, OH, 1910-1990) and the Shawnee Pottery Co. (Zanesville, OH, 1937-1961). Identifying the piece to the pottery requires a thorough examining of the base, including glazed over stilts, yellow ware clay, and glaze versus non-glaze bottoms. Finishes from the early 20th Century were often shiny until 1915 when matte glazes came into vogue. Shiny glazes returned to popularity during the late 1930's through the 1960's.
To decipher the specific manufacturer of a vintage USA marked piece, educate yourself to the identifying marks, including weight of the piece, color, glaze techniques and base finish.