Thank you, Michael G, you certainly know your way around finding patent numbers and long lost engineering tool makers, I  thank you for this information, a very interesting read.

This tool is so precise, as soon as you hold it in your hand, you know its something very special. Interestingly, the inserts have the tops with a slight curve for inside measuring, however the convex curve, unlike a total curve as with a normal micrometer it is only curved at 90 degrees to the locking angled cut out of the insert, this design would increase its accuracy. There was another one for larger holes on ebay, but didn’t bid for it. I wish that I had now. Bye for now. Graeme.

Hi Bernard, yes, a wonderful tool, congratulations. The slot in the box on the right should contain a small adjustment pin for the little hole in the dial. Yours should lay flat, wheras mine sticks up out of a small hole. Wonder how many sizes where made? Graeme.