Notwithstanding any other law to the contrary, any person who is actively employed by the state of Missouri in a position covered by a retirement plan administered by the Missouri state employees’ retirement system and who had nonfederal full-time public employment in the state of Missouri, and who by virtue of such employment was a member of a retirement system or other employer-sponsored retirement plan other than the Missouri state employees’ retirement system but is not vested in such other retirement system or plan, or was not a member of any retirement system or plan, may elect, prior to retirement, to purchase all of the member’s creditable prior service but not to exceed four years for such service in any plan administered by the Missouri state employees’ retirement system in which the person is receiving service credit for active employment or is eligible for a deferred annuity. The purchase shall be effected by the person paying to the Missouri state employees’ retirement system an amount equal to what would have been contributed by the state in his or her behalf had the person been a member for the period for which he or she is electing to purchase credit and had the person’s compensation during such period been the same as the annual salary rate at which the person was initially employed in a position covered by a plan administered by the Missouri state employees’ retirement system or the Missouri department of transportation and highway patrol employees’ retirement system, with the calculations based on the contribution rate in effect on the date of his or her employment under the provisions of the Missouri state employees’ retirement system with simple interest calculated from the date of employment from which the person could first receive creditable service from the Missouri state employees’ retirement system to the date of election to purchase such service. The payment shall be made over a period of not longer than two years, with simple interest on the unpaid balance. In no event shall any individual be eligible to purchase creditable service under this section if such individual after the completion of such purchase has or will receive credit or service under another retirement plan as defined pursuant to section 105.691 for the same time period of service being purchased pursuant to the provisions of this section. The contribution rate for any judge who elects to purchase service for a period prior to July 1, 1998, shall be equal to a contribution rate which would be used if the judicial system were funded on an actuarial basis prior to that date.