All deputies, assistants, clerks, and employes, employed in the office of the collector of delinquent county taxes at the time this act becomes effective, shall be transferred to and become employes of the county treasurer's office. All such deputies, assistants, clerks, and employes shall become eligible to membership in the retirement system established for county employes, and shall be entitled to receive credit in the retirement system as county employes for all years of service in the office of the collector of delinquent taxes upon payment into the retirement fund of a sum equal to the amount such persons would have paid into such fund if they had joined the retirement system at the time the same was established, if then employed in the office of the delinquent tax collector, or if employed first at a later date, then a sum equal to the amount such persons would have paid into such fund from the time they were first so employed.