Unknown Yale key

[Steven Travis]

attached is a pic of a key I am trying to identify, can you help me?

[Dean Nickel]

Post office box key.... from the back side. That key would never get cut and if you didn't pay your bill that key would slide into the lock and block the keyway preventing you from getting into the box.That hook would prevent it from just falling out. If you compare that to a standard para key Yale it should match as the post office locks were reverse.