§ 371. Optional retirement at age fifty-five; alternative plan. a. Any member of the police and fire retirement system, who, while a member of the state employees' retirement system, elected to contribute on a basis of retirement at age fifty-five pursuant to section seventy-one of this chapter and who did not withdraw such election as therein provided shall, contribute to the police and fire retirement system at the rate prescribed by such section, and if such member retires under the provisions of such section his retirement allowance shall be computed according to the provisions of such section.