For sale is a YALE SUPERIOR INTERLOCK KIT S105810Y 11973. This interlock is very clever, because you can only take the key out when the lock is locked. This way, if you leave yourself only one key, you will mechanically insure that only one lock is unlocked. This lets you create an interlock on whatever you want, such as electrical switches. made of solid brass, this has a very high end feel to it.