Padlocks have been used for securing the mail since the eighteenth century. Postmasters and clerks used locks to ensure that, once mail has been placed inside a mailbag, only a postal employee could retrieve it. A variety of postal employees carried keys to these locks, as the bags were often opened while in postal custody.
Brass locks were used on registered and “through” mail that remained locked until reaching its destination post office. Registered mail locks included rotary locks equipped with pin tumbler counters. These brass padlocks were also used for mail traveling on long routes. Only postmasters were provided with the keys and authorized to open “through” trip mail.
Nancy A. Pope, National Postal Museum