The black parts are the connectors used for the holding the feet in place inside the QCW frame. Please see below for pictures showing how to install the feet:
Step 1: remove the cap on the QCW frame as shown below
Step 2: Remove the bolts
Step 3: install the connector for the levelling foot inside the rail opening
Step 4: Attach the levelling feet as the threads of the leveling foot will screw into the connector’s threads locking the foot in place.