I have not seen one like that, but just looking at the design, I could make a guess...
It may be designed to work only with the 5/16 die. The through hole behind the die is 5/16" and is used as a pilot hole to keep the die lined up straight on the workpiece. The end that has the thumbscrew has a hole that a handle would slide into and be secured with the thumbscrew.
It may have been designed for power threading on a lathe, where the die is started by hand (hence the knurling), then the 'handle' is installed and it rests on the lathe bed while power is applied to turn the work.
All of this is just a guess... but it does look like a tool that was made for a particular job that someone was doing a lot of...
-Bear