Did a little more investigation into why the 9 mm mags will not fit into my 4006 magazine-well.

The rear corners of the 59/5906 series magazines have a sharper corner radius on them,
the 4003/4006 are made with a larger radius to them.

The mag-wells also match their perspective magazines, so the 5906 mags interfere with the 4006 mag-well.
Regards,
BM1