Nothing in this chapter shall be construed as applying to the manufacture, storage, sale or use of signals necessary for the safe operation of railroads or other classes of public or private transportation or of illuminating devices for photographic use, nor as applying to the military or naval forces of the United States, or of this state or to peace officers, nor as prohibiting the sale or use of blank cartridges for ceremonial, theatrical, or athletic events, nor as applying to the transportation, sale or use of fireworks solely for agricultural purposes; provided, that the purchaser first secures a written permit to purchase and use fireworks for agricultural purposes only from the state fire marshal, after approval of the county agricultural agent of the county in which the fireworks are to be used, and the fireworks must at all times be kept in possession of the farmer to whom the permit is issued. Such permits and fireworks shall not be transferable. Items sold for agricultural purposes shall be limited to those items that are legal for retail sale and use within this state.