For any project, division of a project, development unit of a project, or supplemental works on a project, under construction on August 4, 1939, or for which appropriations had been made, and in connection with which a repayment contract had not been executed, allocations of costs may be made in accordance with the provisions of section 485h of this title and a repayment contract may be negotiated, in the discretion of the Secretary, (1) pursuant to the authority of subsection (a) of this section or (2) in accordance, as near as may be, with the provisions in section 485h(d) or 485h(e) of this title. In connection with any such project, division, or development unit, on which the majority of the lands involved are public lands of the United States, the Secretary, prior to entering into a repayment contract, may fix a development period for each irrigation block, if any, of not to exceed ten years from and including the first year in which water is delivered for the lands in said block: Provided, That in the event a development period is fixed prior to execution of a repayment contract, execution thereof shall be a condition precedent to delivery of water after the close of the development period. During any such development period water shall be delivered to the lands in the irrigation block involved only on a toll-charge basis, at a charge per annum per acre-foot to be fixed by the Secretary each year and to be collected in advance of delivery of water. Pending negotiation and execution of a repayment contract for any other such project, division, or development unit, water may be delivered for a period of not more than five years from August 4, 1939, on the same toll-charge basis. Any such toll charges collected and which the Secretary determines to be in excess of the cost of operation and maintenance during the toll-charge period shall be credited to the construction cost of the project in the manner determined by the Secretary.