Any desert-land entryman or his assignee entitled to the benefit of section 337 of this title may, if he shall so elect within sixty days from the notice therein provided, pay to the officer designated by the Secretary of the Interior of the local land office the sum of 50 cents per acre for each acre embraced in the entry, and thereafter perfect such entry upon proof that he has upon the tract permanent improvements conducive to the agricultural development thereof of the value of not less than $1.25 per acre, and that he has, in good faith, used the land for agricultural purposes for three years and the payment to the officer, at the time of final proof, of the sum of 75 cents per acre: Provided, That in such case final proof may be submitted at any time within five years from the date of the entryman’s election to proceed as provided in this section, and in the event of failure to perfect the entry as herein provided, all moneys theretofore paid shall be forfeited and the entry canceled: 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.