So I think what Marcus is describing is the standard slotted countersunk screw (2BA) with a small hole/dimple drilled centrally in the slot, and then you hollow grind a screwdriver bit to fit the slot precisely, but when you are grinding the bit leave a central projection to engage with the hole/dimple in the screw, the point of this is when using an electric drill/screwdriver set to a low torque setting you can whiz the screws in without the screwdriver bit skipping out of the screw slot, I sort of get it, but as pulling the clutch apart is not exactly an everyday job I am happy to just screw up the three central screws evenly by hand to full spring compression, and the other 6 screws just spin in easily, then just nip all 9 up FT, the secondary precaution that Marcus uses is basically a home made self adhesive washer made out of Duck tape, the washer can be made up from a few pieces of Duck tape, and made to the same size as the C27, then stuck on and over the C27 and the screw heads, the theory being that if any of the screws start to loosen they can only loosen so far, probably not even half a turn, and this saves any loosened screws cutting a nice neat circular groove on the inside surface of the clutch cover. Hope that all makes sense.