AccuGuide, not to be confused with any personal lubrication products.

That was something they did in the early 90's, maybe '92 or '93, I know that it has been explained far better than I will with this attempt, they patented the term and I think it may have to do with the vertical marks on either side just below and at the back of the feed lips. I know that the original 10mm magazines didn't have them (1990) and the later ones did, so that's how I point to early 90's as to the "when."