This is an antique Westcott No. 80 10-inch adjustable "S" wrench, marked with "Westcott" and "The Keystone Mfg. Co." cast into the handle, with "10 Inch No. 80" and "Buffalo NY USA" cast into the reverse. The overall length is 10.2 inches, and the maximum opening is 1.5 inches. The finish is black paint.
Keystone Manufacturing was a tool maker operating in Buffalo, New York. The company's products included ratchets, ratchet drills, adjustable wrenches, and socket sets. One of their better-known products was the Westcott adjustable "S" wrench, originally produced by the Westcott company but made by Keystone from about 1900 onward.
This wrench is a real beauty and would make a great addition to a vintage tool collection, a garage peg board, or as an accent piece to a steampunk décor.