The crud appears to be media from the media blasting prior to powder coating. Somehow it is imbedded between the threads and baked on-- it doesn't come out with brushing using a metal brush.

The threads themselves are fine, and I was able to thread the BB in all the way. But the resistance to threading was high and whatever crud was in the valleys between the threads caused excessive compression on the cartridge-- enough to feel binding on the bearings when turning the spindle. The BB is an SKF.