A member who retires for service prior to the age of becoming eligible for retirement payments under the federal system and who at the time of retiring is fully insured under the federal system, may, with the approval of the Retirement Board, elect to have his retirement allowance increased prior to such eligible age and reduced after such age by amounts which have equivalent actuarial values. This modification is for the purpose of co-ordinating a member’s retirement allowance with benefits receivable from the federal system.