Any provision in this chapter to the contrary notwithstanding, any employee who was subject to the federal retirement system and was a member of the employees’ retirement system of the State on July 25, 1971, shall be entitled to all benefits which accrued from such membership and shall be permitted to continue membership in the system with all the benefits thereof, including the right to acquire as membership service credit pursuant to § 88-59, the period of service for which the employee was denied membership in the system because of being subject to the federal retirement system, provided that such election shall be made before January 1, 1973.