Can't really tell from the photo, but am thinking maybe it's distorted from zealous tightening in the past, and worn to the size of the screw head...? As far as I can tell there's only two sizes the larger (by my measuring) @ 25/64" and the main bore @17/64" or maybe 9/32", don't have that size......I may be wrong
As it happens, I only know that because was playing with a spare cover yesterday, to rectify the same issue, it seems mine leaks mainly from the drain screw, and some from the chain adjuster screw (Plunger model)...I've done what you did- but with a fibre washer, they are very tricky to grind, and still doesn't work
My next trick, is to countersink the main bore with the 25/64 bit and leave a thin shoulder for the socket cap screw, and add in a 1mm section 6mm 'O' ring.