§ 5. "That, as from the first day of January, nineteen hundred and thirty-eight the unladen weight of a goods vehicle shall, for the purposes of any duty of excise chargeable in respect thereof, be taken to include the weight of any receptacle placed on or in the vehicle for the purpose of the carriage of goods or burden of any description, if any goods or burden are loaded into, carried in and unloaded from the receptacle without the receptacle being removed from the vehicle."