Hi there. Definitely 'SS' - this is the personal Spitfire IX of Wg Cdr R.W.F. Sampson, or 'Sammy' Sampson. This Spit IX was the first of two that he flew marked SS, the second being a low back Spit XVIe in the Spring of 1945. Interestingly, in his book he is clear that the high back, standard, Spitfire was faster than the low back XVIe.
Thanks for the thoughts - I too was plumping for white as the colour. Now I just need to work out the soze of the red inner of the roundel and I shouldnhave the size. Good thinking!!
Simon