These thread taps are made by Beall in the US, and enable a thread to be cut directly into a block of wood so that it can be mounted on a lathe without the need for chuck or faceplate.
It has may applications, and is ideal for creating custom jamb chucks, custom faceplates, vacuum mounting assemblies and so on.
It can also be used to thread directly into the back of a workpiece in a similar way to how a screw chuck would work, but where in this case, the screw chuck is the lathe spindle itself.
Beall have produced a couple of instructive videos about these spindle thread taps, which you can see below.
