Could someone explain what the numeric classifications on the Thater brushes mean. I'm probably about to upgrade to a Thater brush but I would like to know a little more about what I'm buying. For example, the 4292/6 B 28mm two band finest silver tip fan, is obviously a 28mm knot size with a black handle but what does the "4292/6" actually mean? How would a "4125/3" differ? Any help with this would be great. Thanks.