If any water-right applicant or entryman shall have, prior to December 5, 1924, failed to pay any installment of his construction charges when due, there shall be added to the amount unpaid a penalty of 1 per centum thereof, and there shall be added a like penalty of 1 per centum of the amount unpaid on the first day of each month thereafter so long as such default shall have continued: Provided, That the penalty of 1 per centum per month against delinquent accounts, is reduced to one-half of 1 per centum per month, as to all installments which may become due after December 5, 1924.