Was the Mk IX capable of carrying a 'slipper tank'? The
PR Mk IX seems to have been so capable.
From the Pilot's Notes for Spitfire IX, XI and XVI, AP 1565J:
An auxiliary "blister" drop tank of 30, 45 or 90 gallons capacity (on the
PR XI, of 170 gallons capacity) can be fitted under the fuselage; the fuel from these tanks feeds the engine direct and does not replenish the main tanks.