Every credit union, the employes of which are members of the pension fund as hereinabove provided for, shall pay to the board of pensions an amount sufficient to reimburse the board of pensions for the amount of benefits paid employes of such credit union as provided for by the act to which this amendment is a part after receiving credit for all contributions made to the pension fund by employes of such credit union: Provided, however, That where an employe of any credit union formerly worked for the city, the annual pension requirement or other benefits shall be prorated on the basis that the number of years such employe worked for the credit union bears to total years of service in city and credit union employment; and the credit union shall reimburse the board of pensions for that proportionate part of the pension or other benefits paid which the credit union service bears to the total; and the balance of annual pension requirements or other benefits shall be paid by such cities.

Every credit union employe shall have the option of joining or refusing to join the pension fund within twelve months after the effective date of this act or within twelve months of the date when the privilege of joining occurs, whichever is later. Any credit union employe shall receive full credit toward his pension under the act for each year of service as the employe of the credit union upon his producing proof satisfactory to the pension board of the number of years of his service as an employe of the credit union and upon his making back payments at the same rate as if he had been a city employe in a lump sum or by installments as may be determined by the board. If the employe is retired under this act before the back payments have been completed, the annuity shall be reduced by an amount equivalent to the unpaid back payments or deduction increases not anticipated.