I’m sure there is a brief mention of this in Sparey’s book ” the amateurs lathe” I do remember seeing a ML7 fitted with a fabricated version of this for sale some years ago. Mr Sparey also designed a indexing device based around Myford changewheels, castings were at one time available for this. The Sparey gearbox was presumably the forerunner to the kit for the Machin gearbox available from Hemingway kits. Mr Sparey was evidently a bit of a character, he lived in a residential area and his neighbours took him to court to prevent him testing his model I/C engines in the middle of the night!
Edit. Just looked and there is a write up and picture of Mr Sparey ,the night terror himself, on the Lathes website under Third party and other gearboxes, along with some info on the box itself looks like he was on TV too. Wish they had interesting things like that on there now! I might be in a minority of one but I do find the history of model engineering and machine tools interesting.