hard to find all original early 1920's intact antique american industrial "do not converse with the operator" factory sign with vibrant cobalt blue lettering

very rare all original c. 1920's antique american industrial single-sided porcelain enameled factory employee notification sign designed and fabricated by the ready-made sign company, new york city, ny. the diminutive sign is comprised of heavy gauge die cut steel. the robust factory sign features bold cobalt blue lettering against a mostly white background. the top edge of the factory sign contains two holes for mounting and/or hanging within the vicinity of the factory machine operator. measures 6 x 4 inches.