Thomas C. Jefferies, The Postal System of the United States and the New York General Post Office
“ The handling of registered mail differs chiefly from the handling of ordinary mail in the extra care which is taken to safe-guard it. The aim is to record it at the time of receipt, and to thereafter require all persons handling it to account for it as it passes through their hands along its route. Receipts are required at all points, and the letters are forwarded in pouches secured by "rotary locks," provided with certain numbers running in sequence, controlled mechanically, the mechanism being such that the lock cannot be opened without raising the number at which the lock was set. ”
