On and after January 1, 1974, any claim for benefits for death or total disability due to pneumoconiosis shall be filed pursuant to the applicable State workmen’s compensation law, except that during any period when miners or their surviving widows, children, parents, brothers, or sisters, as the case may be, are not covered by a State workmen’s compensation law which provides adequate coverage for pneumoconiosis, and in any case in which benefits based upon eligibility under paragraph (5) of section
921
(c) of this title are involved. they shall be entitled to claim benefits under this part.