In the utility industry, locks of the plunger operated type are often used to protect meters, meter boxes and valves that control the supply of electricity or gas. A suitable "key" or operating tool is provided to service personnel to enable them to open the locks as required. However these keys are often lost or stolen, and over a period of time many of these keys find their way into the hands of unauthorized personnel, who use them to remove and reverse meters, or to short across meter terminals, or to open valves that have been locked closed for nonpayment of utility bills.
Also, utilities that have adjoining territories prefer to have locks and keys that are not interchangeable, so that keys lost by service personnel of one utility cannot be used to open the locks of the adjoining utility.
Various efforts have been made to provide locks that can be opened only with a special key, and keys that cannot be modified to open other locks. For example, in my U.S. Pat. No. 4,252,006 there is illustrated a plunger lock and key in which the engaging means between the fingers of the key and the lock plunger have a configuration such that "generations" of locks and keys may be provided, so that a later "generation" key will open earlier "generation" locks, but will not open later generation locks. However in some cases the keys can be modified by unauthorized persons to open later generation locks.
In my copending application Ser. No. 281,701 filed 07/09/81 there is illustrated a lock and key combination which provides an improved system of locks and keys of various levels, which utilizes a key as disclosed and claimed in this application.