Hi Iain,
Thanks for getting back again. I take it then that the "Ignored armour..." column is effectively "extra" hits that can be taken by a fortified structure (emplacement), before we start eating into its normal, size related hits.
For example a Small Bunker (size 6) as listed on the table would have 1.5x4=6 size related hits, but could take up to 6 initial hits (representing fortification) before its 6 size related hits would start to be blown away.
I'm confirming as the MC rules carry no such info at all of course.
Cheers,
Paul