It looks like contamination. When you did the alcohol wipe to remove the amine blush, if any of it snuck down into the bores for the screws as you wiped, each pass with the cloth would pull a bit of the contamination back over the surface. Did you use a Q-Tip or little "drill bit" of material with DNA to clean out the holes along the way? Phil, I wonder if some bits of waxed shellac got into your stuff at the factory. We can be assured there was some sort of contamination at any rate. What a bummer!