The plastic adapters are drilled for the rivets to attach to the door and on the speaker face for screws.
I really wouldn't use the plastic adapters. I have great overall bass response given the location, BUT snare drums seem to induce a strange panel resonance that is annoying. I blame the plastic adapters for this, there is no dampning between the adapter and the door. I am pi$$ed off as I'm going to strip them down again to fit MDF speaker rings as a potential cure.
<- Winner two pair!