A member of the state employees retirement system who has been in the active full-time employment of another state or states for some past period or periods for which he has received or will receive no retirement benefit or pension and for which he cannot obtain credit for retirement purposes under any section of this chapter may receive credit for such period or periods, not to exceed ten years in the aggregate, by making contributions to the State Employees Retirement Fund, for each year of such prior service, equal to six per cent of his annual rate of compensation when he first became a full-time employee of this state, provided the state or states in which such service was rendered make similar provision for former employees of this state and provided such payment into the state employees retirement system shall be made within one year of his entry into service in this state or on or before July 1, 1966, whichever is later. When a person who has gained credit for such prior service retires, not more than one year of such out-of-state service may be counted for each two years of state service; provided, if such person has purchased more of such service than can be counted, refund of the amount paid on the extra years of service shall be made.