Where services covered by a retirement system established by a political subdivision are included under an agreement under this act and where any laws providing for administration of retirement systems in that class of subdivision do not specifically provide for such cases, the governing body of such subdivision may reduce the basis upon which retirement allowances paid by such retirement system are computed to a proportionate basis which is adjusted for social security benefits. Any such reduction shall apply only to benefits payable after the age at which Federal Social Security benefits are payable and to that portion of such benefits based on wages (as defined in section 2 of this act:¹ Provided, That the total sum to be received monthly from social security and the retirement system following retirement by an employe who is a member of the system at the time of such change shall not be less than the allowance that would be paid by the retirement system in the absence of such agreement. Where the employe contributions are actuarially determined as amounts sufficient to provide a specified portion of the benefits such employe contributions may be correspondingly reduced. The provisions of this section shall not be deemed to restrict any city of the first class in modifying any pension and retirement system covering any officers or employes of such city.