Mo. Rev. Stat. § 143.009
Sections 143.011 to 143.996 shall become effective on January 1, 1973; but it shall apply only with respect to taxable periods beginning on or after January 1, 1973. The repeal of the provisions of chapter 143 shall become effective January 1, 1973, but it shall not affect any taxable periods beginning before January 1, 1973, in any respect, including, but not limited to, the determination of tax, interest, penalties, procedures, and periods of limitations. Notwithstanding the first and second sentences of this section, section 143.471, relating to electing small business corporations, shall apply to such corporations and their shareholders with regard to taxable periods of such corporations ending on or after January 1, 1973. The preceding sentence shall not apply with regard to taxable periods of electing small business corporations beginning before January 1, 1973, if such corporation and all of its shareholders elect to that effect with the director of revenue on or before January 1, 1973. Notwithstanding the first four sentences of this section, a taxpayer who has a fiscal period which includes parts of each of the years 1972 and 1973 may determine his tax and taxable income pursuant to the provisions of sections 143.011 to 143.996 if he files an election to that effect with the director of revenue on or before the due date (including extensions of time) of his return for the taxable period.
§ 143.009, RSMo
Effective 1/1/1973