You'll have my PM but Sparey had drawings or a photo of one that he'd made in the Amateurs Lathe book.
It works on the principle of going in and out of cut. I got a set when I got my Zyto( Noah was about then) and I've sort of lost them. However, another set of drawings are in Ian Bradley's Shaping Machine and Lathe tool book. Mebbe out of print- to where I'm going