(a) If bonds or obligations secured by a trust mortgage have been issued prior to or simultaneously with the recording of the mortgage and the amount of the bonds or obligations are less than the maximum amount secured or which under any contingency may be secured by the trust mortgage, the mortgage may contain a statement of the amount advanced or secured by the mortgage as of the date of recording the mortgage and the mortgage recording taxes payable at recording will be computed only upon the amount so stated to have been advanced or secured.

(b) Where a statement of the amount advanced or secured by a trust mortgage is contained in the mortgage and the tax is computed on such stated amount, the statement shall be at all times binding upon and conclusive against the mortgagee, the holders of any bonds or obligations secured by the mortgage and all persons claiming through the mortgagee any interest in the mortgage or in the mortgaged premises.

(c) If a trust mortgage covers property both within and without the State, the taxable portion of the original advance and any subsequent advances as discussed in section 646.3 of this Part are determined in accordance with Part 649 of this Title.