I'm guessing that 542 and 543 must be Atlas stock #'s for code 83 turnouts and what Mike is looking for is the equivalent in code 100. Mike, see if the part list you have indicates the size of the turnout, #4, #6, so on. If so, that is all you need to know, you can buy a #6 turnout in code 100 or 83. Perhaps someone else will look it up for you and post, I can not take the time right now.
Gary