This is an Underwood Elliot Fisher Company blank stationary envelope. The envelope is sealed and empty. The acidic nature of the glue used on the envelope seems to be leaving dark marks on the back of the envelope. The envelope measures 9.5" (w) x 4.25" (l).
The Underwood company produced typewriters beginning in 1874, the company was purchased and reorganized in 1927 creating the Underwood Elliot Fisher Co. The company was purchased again in 1963 and was renamed the Olivetti Underwood Co. - wikipedia (ALowrie)