John, The 00 and 0 refer to the original thickness of the bronze powder slide. The 0 size was the thinnest (dont have that number handy), and the 00 size was thicker. Other spacer kits and slides were made up to 5/8". These helped to accomadate the larger charges of powder needed for larger cartridges. The 0 and the 00 were the earliest and the most common, these handle the small charges of bullseye powder that were common for Bullseye target shooting. I will check the thickness of those bars later unless someone can chime in here.
John