Section 90C3/4. A former state or metropolitan district police officer, retired before July 1, 1992, who has been retired under this chapter or similar provision or earlier law on account of superannuation after having served in the state or metropolitan district police force for a period of not less than 20 years shall have his retirement allowance increased to an amount not exceeding one-half the rate of regular compensation payable to state police officers holding similar positions, at the time of increasing the allowance, in the comparable grade or classification occupied by the former officer at the time of his retirement.