Interesting shot with the nearest aircraft having what appears to be a 'Type B' fuselage roundel (unless it's just the blue disc over the camouflage, awaiting completion as a 'full' roundel), and those 'wavy' patches, forward of the 'Sky' tail band, on all aircraft.
From spitfiresite.com "According to its archival caption, this image depicts the training of Yugoslav ground crew with the RAF In Italy, prior to the creation of No. 352 (Yugoslav) Squadron"
In this link that patch ahead of the tail band was either covering the flare tube hole or a gas detection patch. Spitfire Odds-n-Ends At the bottom is a link to Britmodeller.com that discusses this and I'm too lazy to look for it WWII
Could be gas patches - I've seen similar shape light coloured patches near the tail on Stirlings. The dull red, doped fabric patch for the flare tube was slightly further forward, and normally square, as shown on Duxford's MH434 below.
The outline can be seen on the nearest aircraft.
I agree with Terry. What is more, the patch can't be for the flare tube because its outlet still can be seen at the proper location. Here the enlarged pic..
Royal Air Force Operations: Airmen Prepared for a Supermarine Spitfire Mark VIII of No. 607 Squadron for a sortie during moonsoon conditions at Mingaladon, Burma. Pic courtesy of Imperial war museum.