Slotted base half inch post holder. The holes running up along the body are intended to be tapped for 1/4-20 thumb screws to lock the post in place. The slot in the base is size for clearance to slide along a 1/4-20 screw mounted into a standard 1" x 1" spacing optical breadboard/table. You should use washers when fastening the base to the table to extend the life of the part.

The base can also be secured to the table via a 3" table clamp that is also posted in my collection.