Registered mail sign
On March 3, 1855, in the act that required prepayment of postage on domestic letters, Congress also authorized the establishment of registered mail service, the first additional special service offered by the Post Office Department. To ensure proper handling of registered mail, all items remained locked in mail pouches while in transit. Only postmasters and clerks assigned especially to registered mail had keys to access registered letters.
Initially, registered mail cost five cents plus the price of postage. However, the fees varied considerably over the next few decades, usually remaining between eight and ten cents through the 1920s. The fee was eight cents, as advertised on this sign, from 1893 to 1909.
Scheele, Carl H. A Short History of the Mail Service. Smithsonian Institution Press: City of Washington, 1970.