I did a countless number of L-brackets years ago for a popular tool manufacturer. They were all "fresh production pieces" but every one of them had this black crap streaking all over them like something that was applied while they were rolling down the line. Is that perhaps the same stuff you're talking about? I always wondered what it was. Not really oily or anything, and it came right off with blasting, but it was just weird marks on top of the metal. Thanks Dan! Your knowledge about so many things is a blessing. :-D