Eric, it's a chaser that uses a master thread to move the carriage via a bar behind the bed. It's become kind of popular with some of the Taig folks. Unimat used similar in the 50's-60's, and some of the early precision makers in the late 19th and early 20th centuries. Pretty neat, once you have a number of masters made up.
I think if you search "Keith Brooks threading" you will be able to get a good look at the works.
Dean