Clause (6) thereof related to gratuity and provided: "Gratuity shall in future be payable by the company in accordance with the following rules: (a)(i) Where, irrespective of the length of his past service, an employee dies in service, or is retired on a medical certificate acceptable to the company, or is retired by the company on reaching the age of superannuation, he shall be entitled to gratuity calculated at the rate of one month 's basic salary for each completed year of service, and pro rata for any partly completed year of his service ,subject to a maximum of 15 months ' basic salary if his service is less than 30 years, together with half of one month 's basic salary for each completed year of service in excess of 30 years and pro rata for any partly completed year of service in excess of 30 years. . . (d) Employees in service prior to Ist January, 1947 may opt, at the time of leaving service, either for: (i) Gratuity calculated in accordance with these rules or in accordance with the current provisions of General Office Order No. 26, whichever he prefers, or (ii) in lieu of gratuity, a retiring allowance calculated in accordance with the current provisions of General Office Order No. 26.