Those plates are a "baffle" under the openings to keep
oil from being thrown up by the valves, by removing them you may experience a lot of
oil weepage on your covers. The passenger side is generally the pvc side, does one of the covers have a riser tube?, that is it if it does. One of the covers should have a "twist-on" opening, that would be the filler.
I'm not an expert by any means just speaking from experience as my brother pulled them out of his Mustang covers and ended up very shortly with
oil film all over his newly refurbished engine and engine compartment.