This is a replacement Pitlock nut for quick release skewers. If you need to replace a solid axle nut, they can be found here.
This is the most critical piece of any Pitlock locking mechanism. There are hundreds of different shapes to the Pitlock coded nuts, so the likelihood of a thief having a key that will fit your nut is next to zero.
Note that this is not a complete locking mechanism, only the coded nut. For a complete locking mechanism setup, you will also need to order the lock housing, a compression washer (either for a seat post skewer or a wheel skewer), a teflon ring and a lock ring.