Hi,
the plate lays loose on top of the filter but very close to the edge. There are 4 holes in it, one in the center for the fixing screw and 3 around to let the oil run through. It's very easy to open the reservoir, just open the butterfly nut, remove the cap. There you find a spring which tightens the plate against the cap. You can remove the spring and the plate. The center screw remains very loose in the reservoir.
...WRe