Stan944

OK, slotting the screws helped. Still, I had to squeeze the assembly and the flat bit (jammed with a couple of hex sockets) in a wise to prevent it from jumping out, and twist the bit with a wrench (it's hard to explain).

Ironically, these thress Philips screws held the compressor valves, not the plate I wanted to remove. I didn't guess because these valves were not visible. It turned out all I had to do was to pry out the plate with the valves still attached onto the other side.
Well, sometimes it takes more work than it should; I feel a bit stupid...

Thanks for the tips, Stan