Let’s consider the FeI first:
The z-axis is fourfold and the x and y axis are also fourfold, if the FeIIa and FeIIb sites are the same, else they are twofold. From this we can conclude that the EFG is zero in the site and that either axis can be used as z-axis of a PAS.
For the FeII sites:
Again considering FeIIa and FeIIb sites are the same, all axis are fourfold, meaning the EFG is zero in the site and that either axis can be used as z-axis of a PAS.