Not to be confused (as I did in earlier post) to the Geometric Die Heads.  After much digging around I have found tons of Geometric Die Heads, a few odd brands but nothing like the GTD one pictured below.

This GTD version seems to be adjustable over a particular range of diameters.  The markings on the chasers (the only ones with it) say 18 18.  Typical Type D chasers are specific to diameter and tpi - say 5/16-18 for instance.  This GTD version is marked 1/2" so basically the 18 18 chasers would be useful for 5/16-18 and not much else I am afraid.

The D type chasers are available, yet to find the J Hook looking ones that are used in the GTD Model.