Mo. Rev. Stat. § 116.220

The secretary of state shall label statutory initiative and referendum measures alphabetically in the order in which they are submitted by petition or in the order in which they are passed by the general assembly. The secretary of state shall label the first as "Proposition A", and so on consecutively through the letter Z, and then begin labeling as "Proposition AA" and so on. A new series of letters shall be started after each general election. In the event a measure is labeled prior to, but not voted on at, the next succeeding general election, the letter assigned to such measure shall not be reassigned until after such measure has been voted on by the people.