The notches were put there to allow the use of a C spanner to help locate accurately if they are a little tight in the upright.
They can be left out if not needed.
In the UK some IVA stations require a secondary fixing point for the mushroom adapter.
A simple grub screw will do.
Personally I think the reason for a secondary fixing is to eliminate the risk of the insert lifting out of the upright if the pinch bolt becomes loose or snaps.
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