Brian, what did the dykem show?
There are quite a few nicks on the backplate so it might have been mistreated. Or maybe the backplate was not machined well enough to fit the spindle properly.
Either way why not remove the back plate and chuck it in the 4 jaw. Check concentricity and square of the register surfaces and threads. Single point the internal threads a bit to get better clearance and concentricity. The thread fit is not critical. The register surfaces are. To me this a great learning opportunity for you.
There are quite a few nicks on the backplate so it might have been mistreated. Or maybe the backplate was not machined well enough to fit the spindle properly.
Either way why not remove the back plate and chuck it in the 4 jaw. Check concentricity and square of the register surfaces and threads. Single point the internal threads a bit to get better clearance and concentricity. The thread fit is not critical. The register surfaces are. To me this a great learning opportunity for you.