"(b)    Notwithstanding the provisions of the preceding paragraph, a member may be allowed to retire after rendering a total service of thirty years, regardless of age, the retiring employee to receive a monthly annuity for life, but the benefit for service rendered after June sixteen, nineteen hundred and fifty-one, shall be whatever amount of annuity can be purchased by the accumulated government and personal contributions to the credit of a member plus interest allowed by the system on the date of retirement. Said annuity shall be computed in accordance with the mortality table and the rate of interest adopted by the system. This benefit shall be in addition to the benefit for service rendered prior to June sixteen, nineteen hundred and fifty-one as provided in section eleven (A) of this Act.

"(c)    Retirement is likewise allowed to a member, regardless of age, who has rendered at least twenty years of service. The benefit shall, in addition to the return of his personal contributions plus interest, be only a gratuity equivalent to one month salary for every year of service, based on the highest rate received, but not to exceed twenty-four months. This gratuity is payable by the employer or office concerned which is hereby authorized to provide the necessary appropriation or pay the same from savings in its appropriations."