Mo. Rev. Stat. § 144.525
Notwithstanding any other provision of law, the amount of any state and local sales taxes due on the purchase of a motor vehicle, trailer, boat or outboard motor required to be registered under the provisions of sections 301.001* to 301.660 and sections 306.010 to 306.900 shall be computed on the rate of such taxes in effect on the date the purchaser submits application for a certificate of ownership to the director of revenue; except that, in the case of a sale at retail of an outboard motor by a retail business which is not required to be registered under the provisions of section 301.251**, the amount of state and local taxes due shall be computed on the rate of such taxes in effect as of the calendar date of the retail sale.
§ 144.525, RSMo