I did not check the books, but in general for shields several explanations are often given.
spirits / ancestors...
One thing is sure, It is meant to help the one behind the shield
The mouth of this spirit is formed by 2 aso's facing each other.
This one has hornbills that appear to be used to form the eyebrows.
I am not sure when this appearred on shields, but I have a small dance shield that shows the same feature of Hornbills above the eyes.
Seeing them as "eyebrowes" may of course be my western point of view.
They might well be placed above the eyes/face, because they symbolise the "upperworld" and of course the aso's beneath representing the underworld.
Here some pics of the dance shield I bought in Sarawak in 1997.