Thanks, I found it earlier. It only really gives bolt centres so I measured my Chuck and the figures don’t exactly tally with that drawing. With my C jaws set at 70mm OD as suggested by Axminster I can say that the register on the Chuck is appropriately 66mm OD and 60mm ID. The register is actually very slightly under 3mm wide and the corresponding recess on the accessory jaws seems slightly over 3mm. The bolt centres appear to be 19mm. I’m going to make some pins to fit the holes in the accessory jaws to measure the distance to the register as the jaws proper move about a bit too much to take an accurate measurement. Absolute accuracy isn’t needed but I may as well get it as accurate as I can.