Ref the diagram in yr post Morris, just to add complexity to this....my chain case on my 56 has just the filler cap not a clutch adjuster cap, and the first of the smaller screws has a hole in the top of the PCC "bulge" through which the screw goes, so the oil has to reach the top of the bulge before it comes out with the screw removed. That screw I reckon is the 'fill level" screw on my case and the 2nd bolt has the "bulge" cutaway which I reckon makes it the drain screw.
So that makes my case different from the diagram. But then maybe my case is off a B31 or something! Or someone has modified it. The bulges on the bottom are quite chewed where the chain has hit them, and there is some welding on the bulge where the 3rd screw fits.
The haynes manual has the same diagram by the way.
The manual also says there is just 95cc of oil in the case but I've found it needs more like 200cc otherwise the chain runs dry.