Hi all,
I was puzzled by this photo and the two small blisters visible where the upper wing roundels are. Another photo of the same a/c shows similar (but shorter) blisters below the wing.
Can anyone help me to identify the nature of this modification? Judging from the serial number, the aircraft appears to be a factory-fresh Spit Mk. VC of the earliest Supermarine production. I'd be grateful for any comments.


(Image source:
http://www.spitfiresite.com/photos/historic/2009/03/aa963-in-united-states-colour-photos.html)