I went to cast up some 319295 up with my new-old mold. After some measuring and mathification, it seemed pretty clear that (IMHO) the lengths, depths, and widths were really not too conducive to the wave washers from Ace. Nor was the supplied split lockwasher making me hug myself with contentment.
So, delving into long suppressed memories, I took the lockwasher to the abrasive belt and smoothed off one end so it was smooth against the sprue plate. Reassembled, seems to hold compression well, the sprue screw hasn't moved AFAICT. I left the top end rough. If it digs into the bottom of the screw head, so much the better.
PS. It may be that a Booliteer made the suggestion on smoothing the lockwasher long ago...