Rules that Don't Exist

A tz newbie question: What happens when switching to a named rule, but no such rule exists? The Europe/Zurich example in zic.8.txt has an example of that (switches to Swiss rules in 1894, but there are no Swiss rules before 1940), but doesn't say what to do. I can make sense of that by making what seems to be the reasonable assumption that SAVE is zero and LETTER/S is null. Is that the intent? If so, then is it also true that, in America/Chicago, we should use "CT" rather than "CST" from 1883 Nov 18 12:09:24 to the last Sunday in March of 1918? Or is there something I'm missing? Thanks, --Bill Seymour