This 1 x 8 tpi thread tap is made by Beall in the US, and enables 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 many 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 this spindle thread tap, which you can see below.
Instructions are supplied with each tap.