Mo. Rev. Stat. § 476.524
Any judge as defined in section 476.515 who had performed active service in the United States Army, Air Force, Navy, Marine Corps, Army or Air National Guard, Coast Guard, or any reserve component thereof prior to holding office, may elect prior to retirement, to purchase all of his or her creditable prior service equivalent to such service in the Armed Forces, but not to exceed four years, if he or she is not receiving and is not eligible to receive retirement credits or benefits from any other public or private retirement plan for the service to be purchased, and an affidavit so stating shall be filed by the judge with the Missouri state employees' retirement system. However, if the judge is eligible to receive retirement credits in a United States military service retirement system, he or she shall be permitted to purchase creditable prior service equivalent to his or her service in the Armed Forces but not to exceed four years, any other provision of law to the contrary notwithstanding. The purchase shall be effected by the judge's submission of the appropriate documentation verifying the judge's dates of active service and by paying to the Missouri state employees' retirement system an amount equal to what would have been contributed by the state on the judge's behalf had the judge been a judge for the period for which the judge is electing to purchase credit and had the judge's compensation during such period of membership been the same as the annual salary rate at which the judge was initially employed, with the calculations based on the assumed or actual contribution rate in effect on the date of employment with simple interest calculated from the initial date of employment of the judge to the date of election pursuant to this section. The payment shall be made over a period of not longer than two years, measured from the date of election, with simple interest on the unpaid balance. All payments for purchase of service pursuant to this section shall be set aside in a reserve fund for funding of said benefits. Payments made for such creditable prior service pursuant to this section shall be treated by the Missouri state employees' retirement system as would contributions made by the state and shall not be subject to any prohibition on member contributions or refund provisions in effect at the time of enactment of this section.
§ 476.524, RSMo