very unique single-sided late 1920's american antique industrial factory instructional sign likely fabricated by the ready-made sign co., new yorkcity, ny. the deep cobalt blue porcelain enameled finish remains in great shape. the sign is comprised of die cut steel with four pre-drilled holes for mounting flush against a flat surface. the sign reads, "keep this window closed when exhauster is operating." salvaged from a chicago area factory. measures 13 x 3 inches.