Any member who was formerly an employee of the Georgia Warm Springs Foundation but who became an employee of the state under the terms of an agreement, effective November 1, 1973, between the state and the Georgia Warm Springs Foundation and who became a member of the retirement system shall be eligible to receive credit for previous employment service with the Georgia Warm Springs Foundation, upon payment to the retirement system of his accrued liability, as determined by the board of trustees. This option must be exercised by such member not later than December 31, 1974. Any member electing this option may purchase all or any portion of his previous employment service with the Georgia Warm Springs Foundation and full payment for the accrued liability of such service shall be made at the time of such election. Each such member shall be allowed one election and no additional service may subsequently be purchased.