Shield cover depicted on monument??

[Peroni]

What scene from the column is that from? What are the Auxilia doing?

They are taken from Scene CXXXI and they are the foremost of a group of soldiers crossing a river on a bridge. So, it would seem quite appropriate for the guy to have his shield covered whilst marching.

Let us not forget that at least seven sculptors styles have been identified on the Column, and each would render the figures in their own style.

That's why we see differences in the armour and helmet types depicted. Why shouldn't it be the same for shields? Some sculptors show the inner face of the shield with a very wide band/outer edge (see Sulla Felix's pics earlier in the thread) whilst others show a narrow line like the edging we see in the archaeological record.