I replaced my clutch rubbers on the 400/4 yesterday.
Max sent me six csk screws (allan key head) with the kit. He also advised me to countersink the retaining plate and to use green loctite on the threads and to stake the ends of the threads. I've done as advised and although the top of the csk screw heads are pround of the retaining plate they are about the same height as the rivet heads were. The proof will be when it runs but it all appears to be OK. The old rubbers were split, shrunk and rock hard with a lot of movement on the gear as a result.