The numbered, metal disk served as a key check. A chain loops through a small hole in the disk, inscribed with "1798" and "U.S.P.O.D." in the opposite direction. The reverse side is blank.
Keys were issued to employees and the number on the key check corresponded to their identification or badge number. Checks were primarily used for vehicle and collection box keys. The worker left the check on the rack for each set of keys assigned, and upon returning at the end of the tour the keys were turned in and the check retrieved.
Early Post Office Department examples are made of brass, later aluminum was used. Some US Postal Service checks have the vehicle serial number stamped onto the disk, which is attached to the key ring for easy identification.