As supplied from the factory the RCU and SCU were supplied with slotted screws and Torx screws for the trim plate. I think the Torx were T-15. It is possible the screws were replaced with tamper proof types.
Those screws go through the device mounting plate to the box holes. Drilling them out will not hit any circuit board or other components. The device mounting plate is held to the box by two other screws that will become visible when the trim plate is removed. Below are two drawings pulled from RCU/SCU documentation. The top one shows the boxes they can mount in and the lower one is an exploded view of how it assembles.
Here are Dropbox links to the complete documents.
