No retirement allowance or pension payable under the employees’ retirement system shall be increased by any bonus for any retirant or beneficiary unless the person for whose service the pension is payable has had sufficient service to qualify for the minimum service retirement allowance except as provided in [paragraph] (1) of section 88-11; provided that this provision shall not operate to increase the pension of any person who was receiving a pensioner’s bonus on July 1, 1951, without having met the minimum service requirements but such person shall continue to receive the pension the person was receiving on June 30, 1955.