Never Refill 1 lb., Propane Bottles (DOT-39 cylinders)[4:21]:

The public is cautioned to never refill DOT 39 cylinders, i.e., 1 lb., cylinders used for camping. DOT 39 cylinders, of any size, are strictly non-refillable. Hazmat incidents involving refilled DOT 39 cylinders have occurred, including one fatality.