valve-key

from The Century Dictionary.

• noun A form of socket-wrench having a T-head, or other shape resembling the winding-key of a clock, which fits the squared or prismatic shape on the end of a valve spindle. It is designed for use where the valve should not be opened or shut except by one duly authorized and possessing such a key or wrench.
• noun A key fitting the lock which is on the casing surrounding a safety-valve or other valve, and by which tampering with the valve is prevented.