40 ILCS 5/16-149.3

An annuitant who retired on account of disability prior to July 22, 1947 may have the amount of his annual disability retirement annuity increased to $400 upon contributing an amount equal to the lesser of: (1) one month's current annuity, or (2) the difference between the current annual annuity and $400. The increase in disability retirement annuity provided under this Section shall be effective as of the first month following receipt of the required contribution from the annuitant.