When the incumbent of any elective office in this state is unable to discharge the duties of the incumbent’s office by reason of service in the armed forces of the United States, an acting official who shall have the powers of the incumbent must be appointed in the same manner that an appointment would be made in case of a vacancy in office, the appointment being made from a list of three names which must be submitted by the incumbent to the appointing body or officer within thirty days after leaving for the service. Provided, however, in the office of state’s attorney the names of practicing attorneys residing in adjoining counties may be included if there are less than three practicing attorneys residing in such county in which the appointment is to be made. If the incumbent has already entered the military service the incumbent shall, within ten days after the passage and approval of this section, file a list of three names with the appointing body or officer. If the incumbent fails to submit a list of names, the appointing body or officer shall make the appointment of the acting official without regard to the incumbent’s wishes; provided, however, that in the filling of such vacancy in the office of state’s attorney, a practicing attorney from an adjoining county may be named to fill such vacancy if there are less than three practicing attorneys residing in the county where such vacancy is to be filled. Provided, further, the acting official shall serve, and the acting official’s tenure of office must be terminated immediately upon the incumbent filing a “notice of return” with the secretary of state in instances in which it is a state official, or with the county auditor when the incumbent is an official of the county or any of its subdivisions. This section is hereby declared to be retroactive and all appointments to vacancies heretofore made in the manner herein provided are hereby validated.