Direct AP & HE fire at bunkers

Couldn't find any info on DIRECT fire on bunkers. For example, a German Pzkw IV E or F1 has an AP rating of '4' at closer ranges for it's 75mm L24 gun. Fine, but what would be used for the Armor Rating of the bunker? Same question for the '2' rating for the HE. What is it compared too?

It could just be me.

Thanks for the help.

Bunkers don't have an armor rating as such and AP shells are all but useless against them - this is precisely why the 7.5cm L/24 and similar guns were deployed for close support. What you are actually shooting at is the target inside the bunker, which then gets its appropriate cover save. The development of more effective HE rounds in dual purpose guns later in the war made these smaller tank-mounted howitzers obsolete