Mo. Rev. Stat. § 115.451
If authorized by the election authority, the election judges may read and record votes before the close of the polls. If so authorized, the election judges shall use one ballot box for the deposit of ballots during the first hour of voting. At the end of the hour, the receiving judges shall deliver the ballot box to the counting and recording judges, who shall give the receiving judges a second empty ballot box. The second ballot box shall be shown to be empty and locked in the manner provided in section 115.423. The second ballot box shall not be opened or removed from public view from the time it is shown to be empty until the time the polls close or it is removed for counting pursuant to this section. The ballot box containing the voted ballots shall be taken to a private area within the polling place, and the ballots shall be read and recorded in the manner provided by section 115.449. In no case shall ballot boxes be switched at less than one hour intervals and then only if twenty-five or more ballots have been voted during the hour.
§ 115.451, RSMo
Effective 1/1/1978