I'm in the process of lapping the block by hand. It's flat with no measurable low spots, but I'm going for as close to mirror finish as I can get being careful not to create any low-spots.
I took the stopper out of the plastic, and lo and behold...it does have viton on it. Still haven't settled on whether to strip it off and use a thicker sealant. It all depends on how my deck looks when I'm finished. So freakin' tedious it's unbelievable. My hands and shoulders are killing me from lapping this thing.
The temptation is to say "fuck it, good enough", but I know what that attitude will get me, so off I go to the garage...sigh...ooooh yeah....