Mo. Rev. Stat. § 87.185
If, at any time since first becoming a member of the retirement system, a member has served in the Armed Forces of the United States, in any war or period of armed hostilities between the Armed Forces of the United States and those of a foreign power, and is subsequently reinstated as a firefighter within ninety days after his or her discharge, the member shall be granted credit for such service as if his or her service in the fire department of the city had not been interrupted by his or her induction into the Armed Forces of the United States, and as if the member had made the required contributions during such service. If earnable compensation is needed for such period in computation of benefits it shall be calculated on the basis of the compensation payable to the officers of his or her rank during the period of his or her absence.
§ 87.185, RSMo