Any employee in the service, after having made contributions covering a period of 10 or more years to the annuity and benefit fund herein provided for, may elect to pay for and receive credit for all annuity purposes for service theretofore rendered by the employee to the Chicago Transit Authority created by the Metropolitan Transit Authority Act or its predecessor public utilities; provided that the last 5 years of service prior to retirement on annuity shall have been as an employee of the City and a contributor to this Fund. Such service credit may be paid for and granted on the same basis and conditions as are applicable in the case of employees who make payment for past service under the provisions of Section 8-230, but on the assumption that the employee’s salary throughout all of his or her service with the Authority or its predecessor public utilities was at the rate of the employee’s salary at the later of the date of his or her entrance or reentrance into the service as a municipal employee, as applicable. In no event, however, shall such service be credited if the employee has not forfeited and relinquished pension credit for service covering such period under any pension or retirement plan applicable to the Authority or its predecessor public utilities and instituted and maintained by the Authority or its predecessor public utilities for the benefit of its employees.