Tenn. Code § 49-1515
Subject to the above restrictions and to such other rules and restrictions as the board of trustees may adopt, the board of trustees shall verify, as soon as practicable after the filing of such statements of service, the prior service therein claimed. In no case shall such prior service become creditable unless evidence thereof satisfactory to the board of trustees has been submitted to it prior to July 1, 1959, except as provided in §§ 49-1518, 49-1542 and 49-1552.
In lieu of determination of the actual compensation of the members that was received during such periods of prior service, where such actual compensation is not otherwise reasonably ascertainable, the board of trustees may use for the purpose of this chapter the compensation rates which will be determined by the average salary of the members for five (5) years immediately preceding the date this system became operative as the records show the members actually received.
T.C.A. § 49-1515