The Exactions never had inserts.
Exacton (I think) made their wheels from Aluminium Bronze. A fairly strong, corrosion resistant alloy that machined well, but is VERY difficult to weld, even if the matching alloy rods are available. ( I know this, from 'sailors' braking the propellers of their outboard motors ). Otherwise, if all four holes could be welded up, it would be quite a simple task, using the circumference, to set up a big enough milling machine to bore four holes where they ought to be.