Not later than ninety days after
October 27, 1972, the compensation to which an employee or his survivor is entitled due to total permanent disability or death which commenced or occurred prior to
October 27, 1972, shall be adjusted. The amount of such adjustment shall be determined in accordance with regulations of the Secretary by designating as the employee’s average weekly wage the applicable national average weekly wage determined under
section 906(b) of this title and (A) computing the compensation to which such employee or survivor would be entitled if the disabling injury or death had occurred on the day following
October 27, 1972, and (B) subtracting therefrom the compensation to which such employee or survivor was entitled on
October 27, 1972; except that no such employee or survivor shall receive total compensation amounting to less than that to which he was entitled on
October 27, 1972. Notwithstanding the foregoing sentence, where such an employee or his survivor was awarded compensation as the result of death or permanent total disability at less than the maximum rate that was provided in this chapter at the time of the injury which resulted in the death or disability, then his average weekly wage shall be determined by increasing his average weekly wage at the time of such injury by the percentage which the applicable national average weekly wage has increased between the year in which the injury occurred and the first day of the first month following
October 27, 1972. Where such injury occurred prior to 1947, the Secretary shall determine, on the basis of such economic data as he deems relevant, the amount by which the employee’s average weekly wage shall be increased for the pre-1947 period.