On December 1, 1911, the US Post Office Department introduced the only registration stamp ever issued.  It prepaid registration fees and guaranteed special care and handling of a letter or package.  The stamp was valid towards the registration fee only, and could not be used as postage.

The new stamps were intended to simplify the registration process, but ended up confusing customers and postal clerks.  People mistook the stamp for regular postage.  As a result, further printing was discontinued in 1913, while the remaining stock was still used.  Though often overlooked by collectors, the expertly engraved US #F1 is one of the most beautifully designed Back-of-the-Book stamps!