A member's prior service accumulations shall be equal to the amount of the contributions he would have made with respect to his earnable compensation for such service had the retirement system then been in operation, together with regular interest on such amounts to January 1, 1945, at the rate of regular interest in effect on that date. Service in a school system in this state which operated a local retirement fund shall be included as prior service for purposes of calculating prior service accumulations. In determining the earnable compensation of any member during his prior service, the board of trustees, in lieu of the actual compensation received by the member during his prior service, may use the rates of compensation which, if they had progressed during such prior service in accordance with the tables adopted by the board of trustees under Code Section 47-3-23, would have resulted in the same average final compensation on January 1, 1945, as the records show the member actually received.