An ad valorem tax of two mills on each one dollar of the total assessed valuation of all the taxable property in each county or counties in the State of Mississippi, in which there is located a harbor or port of entry where commodities are exported to foreign nations, and where there is maintained a channel and/or harbor or port to a depth of not less than twenty feet, be, and the same is hereby, levied on all said taxable property, in or for each year in which the principal of or interest on any bonds or other obligations issued by any municipality pursuant to this article becomes due. The receipts from said two-mill tax shall be withheld by the tax collector of said county, and/or by any other tax collecting agency authorized by law for the collection of said taxes, from receipts from state ad valorem taxes now in effect or which may be hereafter levied, so long as the state ad valorem taxes shall be not less than the two-mill tax herein levied. However, if no state ad valorem taxes equal to or greater than the said two-mill tax herein levied is now or shall be hereafter levied, then and in that event, the said two-mill tax herein levied shall continue to be levied and collected as herein provided in each such county or counties in or for each year in which the principal of or interest on any bonds or other obligations issued by any municipality pursuant to this article becomes due. The tax collector, and/or any other tax collecting agency authorized by law for the collection of said taxes, shall pay over all moneys collected or to be collected as receipts from said two-mill tax to any trustee or successor thereto established as hereinafter in this article provided, and in the event that there is no such trustee, then said tax collector, and/or any other tax collecting agency authorized by law for collection of said taxes, shall pay over all such moneys into the county depository of each such county to the credit of a fund which shall be known as a port fund. Any such moneys so paid into the county depository of each such county to the credit of said port fund may be expended at the direction of the port commission, appointed for any port of entry as designated in chapter 1 of this title. But in no county within the terms of this article shall there be withheld from the state treasury under the provisions of this article and Article 1 of this chapter, for any one year an amount in excess of the receipts from said two-mill tax. The provisions of this article shall be deemed to be a contract with the holders of any bonds or other obligations issued pursuant to this article.