Inserts for the coupling body 4604-050, note these inserts are sold individually allowing you to mix and match to suit your application.
We have also experimented with these inserts by pushing them into our plastic wheels with good results. Simply drill a 7.5mm hole through the plastic hub of a wheel and force these inserts in, one on each side. This allows up to a 6mm axle to be used with the wheel and gripped using the supplied grub screws.