References in the foregoing Table to pounds of weight shall be construed as references to pounds avoirdupois; and the Minister may by regulations prescribe the manner in which the weight of an aircraft fully loaded is to be ascertained for the purposes of this Schedule, and direct that, in the case of an aircraft of any particular description, such document as may be specified in the regulations, being a document which purports to show the weight of the aircraft fully loaded, shall be evidence of that weight