Packing Instruction 213 is used for UN 1044 Fire Extinguisher. The maximum for PAX aircraft is 75kg, and for CAO is 150kg. Both are the net quantity of the compressed gas or liquefied gas contained in the cylinder. It is not the gross weight of the cylinder.
For Overpacks, you will enter as: 1 Cylinder x 2.5kg OVERPACK USED.