Found this beast at a yard sale recently. Have seen pictures of them before, but never had one in hand.
This lock takes a standard Master M1 key blank.
As the title stated, this is for locking trailer hitches. (Fifth wheel hitches). The flat part end of the kingpin fits into here...
When locked, the plug is turned and blocks this hole.
Turning the key...
And unlocked...
Then use a screwdriver to turn the locking bolt, which is threaded down into the kingpin.
When the bolt is firmly in place holding the trailer kingpin, then you remove the screwdriver and turn the key back to the locked position.
This was made back when no Master Lock had any security pins, so it picks very easily. Got it for the uniqueness of the lock. It is very large, too.
Gordon
