cdboot, mfs_root and mount_cd9660

For reasons I wont bore people with, I am trying to produce a bootable CD
using /boot from 5.2 (minus kernel and modules) boot a 4.9 kernel and
mfs_root.

This works fine however once booted trying to mount the iso9660 fs from the
CD produces the error:

cd9660: /dev/acd0c: Invalid argument

booting the same kernel and mfs_root bits from a 4.9 pxeboot via tftp I am able
to succesfully mount the same CD on the same hardware (DL 360 G1)

Anyone know what I am doing wrong?