Contributions by city for duty and occupational disease disability benefits and supplemental annuity. In lieu of salary deductions for annuity purposes, the city shall contribute the required amounts for any period during which a policeman receives a duty disability benefit or occupational disease disability benefit. The contributions shall be credited to the disabled policeman and shall be regarded for all purposes hereof as sums deducted from his salary.
     The city shall also contribute all amounts ordinarily contributed by it for annuity purposes for the policeman as though he were in active discharge of his duties during such disability.
     To provide supplemental annuity, the city shall contribute such equal sums annually, from the date of the policeman’s death, which if improved by interest will be sufficient, when payment of compensation annuity ceases, to provide supplemental annuity to the widow for life.