Unloading any product from an officially sealed railroad car, truck, or other means of conveyance containing any unmarked product or loading any product or any other commodity in the means of conveyance while en route from one official establishment to another official establishment is not permitted, except that product transported under §325.5 from one official establishment to another for further processing may be unloaded and stored in transit at any approved warehouse which is operated under the identification service provided under the regulations in part 350 of subchapter B of this chapter and which has railroad facilities or a receiving dock for unloading the product directly into such warehouse: Provided, That the product is stored in rooms which are of such size and type as will not result in adulteration or misbranding of the product: And provided further, That the product is transported to and from such warehouse, and under official seal as provided in §325.5 and stored in such rooms at such warehouse.