40 ILCS 5/11-171
Any employee in service on July 1, 1947, may elect to make an additional contribution while in service which shall not exceed 7/13 of the sum accumulated for age and service annuity plus interest on July 1, 1947, or at age 65, if he attained such age prior thereto. The time and manner of making such additional contributions shall be prescribed by the board.
Concurrently with each such additional contribution, the city shall contribute 1 and 4/10 times the additional contribution.
These contributions shall be improved at interest at the effective rate, in like manner as other employee and city contributions; provided, that the employee, while in service, may request a refund of all or any part of such contributions, without interest, or shall have them refunded to him, without interest when he retires on annuity or to his widow, if and to the extent they do not serve to increase the annuity otherwise payable to him or to his widow.
By such refund the employee or his widow surrenders and forfeits all rights which might otherwise have accrued by virtue of any amount so refunded, including related city contributions.
40 ILCS 5/11-171