Hi Howard,
I get you now.
Its the Mic stop type of design that I was hoping for with the clamp, but the Clarke bed isnt really suited with only 6mm of recess (which unfortunately has radius) at the rear to hold onto. It would find without radius.
As long as the magnets hold it which I'm confident they will, it'll be OK and easy to remove in case of tailstock use needed. It may even be better that its not fastened, as it might save the DTI/Vernier in case of accidental Tailstock bashing,
Thanks again for all advice