The locking jaw should close around which part of the kingpin?

Step 13: Inspect the Coupling:
• Use a flashlight, if necessary.
• Make sure there is no space between the upper and the lower fifth-wheel. If there is space, something is wrong (the kingpin may be on top of the closed fifth-wheel jaws, and the trailer would come loose very easily).
• Go under the trailer and look into the back of the fifth-wheel. Make sure the fifth-wheel jaws have closed around the shank of the kingpin.
• Check that the locking lever is in the “lock” position.
• Check that the safety latch is in position over the locking lever. (On some fifth-wheels the catch must be put in place by hand.)
• If the coupling is not right, do not drive the coupled unit; get it fixed.
[California Commercial Driver Handbook]