This is a knob for the bottle jack on an Acdelco 2 ton engine hoist however it should fit many others. There are dimensions of the hole in the pictures to verify. It pushes on with a small amount of force so it can't fall off, and pulls off without any trouble.
For installation turn the flattened bar in the pressure release until the flat side is parallel with the pressure release rod. Then push the knob on. It will not go on unless the bar in a specific orientation.
The Cad file is available for download and modification incase it's needed.