3If, after consultation with the Board of Trade, it appears to the Commissioners that the fittings and other equipment supplied with any vessel or other structure include any items the supply of which would not in the ordinary course of events be undertaken by a person building such a vessel or structure for delivery to another as that other's property, the price or, as the case may be, open market value referred to in the said subsection (3) shall be treated for the purposes of that subsection as reduced by an amount equal to the open market value of the items in question ; and the provisions of paragraph 1 of this Schedule shall apply for the purpose of determining that value, subject to any necessary modifications.