Attached find a photocopy/sketch of the clamp bolt and its nut.
The "ramp" part that bears on the ways and the not shown "top view" is the same width as the 9/16" bolt it transitions into.
Note that the thread is not a US standard. Rather this is Special USS although the head of the thread seems "nicked off." I don't think this is Acme thread form. Nor does the application require maintaining the original thread count OR form.
A 9/16" "coupler nut" may give you a good start - although 9/16" nuts are usually 7/8" across flats in standard pattern. Coupler nuts may be routinely smaller across flats.
Hope this helps,
Joe in NH