Where it shall be made to appear to the satisfaction of the Secretary of the Interior, under rules and regulations to be prescribed by him, with reference to any lawful pending desert-land entry made prior to March 4, 1915, under which the entryman or his duly qualified assignee under an assignment made prior to March 4, 1915, has, in good faith, expended the sum of $3 per acre in the attempt to effect reclamation of the land, that there is no reasonable prospect that, if the extension allowed by section 335 of this title or any law existing prior to March 4, 1915, were granted, he would be able to secure water sufficient to effect reclamation of the irrigable land in his entry or any legal subdivision thereof, the Secretary of the Interior may, in his discretion, allow such entryman or assignee five years from notice within which to perfect the entry in the manner required of a homestead entryman: Provided, That in cases where such entries have been assigned prior to March 4, 1915, the assignees shall, if otherwise qualified, be entitled to the benefit hereof.