What is it?

The whole object, with its legs apart. In this state, it is 2 feet 6¼ inches tall, and the ends of its legs are 1 foot 2 inches apart.
The upper part of it. The white plastic panel reads

                    "Crescent"
               PRO. PAT. No. 80 456

This appears to relate to nothing recorded by the UK or US patent offices.
Detail of the head. The hook-shaped bit is 1¼ inches wide, and padded with (now frayed) leather inside and out.
In its other position, with its legs together. In this state its total length is 2 feet 9½.

When its legs are together, its "elbow" b comes out. To get them apart again, you push on thr elbow.

Turning the knob a operates the screw, gripping whatever it is that is held inside the hook.