text: This invention relates to a security lock and, in particular, to a lock for vaults requiring more than one key for operation of the lock.
  Safe deposit or lock boxes generally require two keys. One of the keys is retained by the bank or financial institution. The second key is retained by the customer who rents the box or vault. Before the box or vault can be opened, it is necessary to insert both of the keys in the lock and turn them in a prescribed sequential fashion. Failure to use the customer's key will prevent operation of the lock, thereby insuring that only the customer will have access to the lock box. Failure to use the bank or guard key will prevent unauthorized entry into the lock box. That is, the person who is to enter the lock box must properly identify himself to the bank security personnel and then be accompanied by such personnel during the opening of the lock box.
  Dual key lock constructions have been available for many years. An early patent disclosing such a construction is U.S. Pat. No. 853,486 patented May 14, 1907 by H. P. Townsend. It is desirable that such locks be of solid and secure construction. It is also desirable that such locks have the capability of multiple combinations. Duplicate keys or identical keys for different locks should be avoided. Additionally, such locks should be constructed, if possible, in a manner which will permit changing the lock combination with ease and maximum security. Such a change would be needed, for example, whenever rental of a lock box is terminated by one customer and begun by a new customer. Preferably, the new customer should have a different lock box combination or key to insure that the key which that customer will be using is unique. Another feature desired in a lock box security unit is the inclusion of a bolt which moves longitudinally into engagement with the strike associated with the lock box and includes a dead lock feature. These are some of the objectives and criteria for a high security lock box lock construction.