the rule which has the letter o with a slash and the zero without was used at ibm and a few other early mainframe makers; this is even more of a problem for scandinavians, because it looks like two of their letters at the same time. some burroughs/unisys computers display a zero with a backwards slash. and yet another convention common on early line printers left zero without any extra dots or slashes but added a tail or hook to the letter o so that it resembled an inverted q or cursive capital letter o.