Hi Patch
Full details of an excellent 4 way indexable tool post are in George Thomas “Model engineers workshop manual”, I believe he gives details for both the ML7 and S7, though I am unable to check my copy. Perhaps someone else can confirm this.
I modified his design and built several of these tool posts to fit non-Myford lathes, including a simplified non-indexing version, and as with all his designs, with careful work you end up with a quality product.
Dave