Hi...
For what it's worth, I think A) the fuselage codes are slightly larger than 'standard' on B3+K 'S', and B) the last letter
is an 'S', but being larger than usual, the lower right curve of the 'S' was perhaps painted over the rear edge of the wing root fairing, which seems to be missing from this plane, given the slightly lighter shade of paint where the fairing would've been... just a thought...
.
Regards...
Paul