Kaki
I spontanously read SD code, and concluded it looks like a photo taken during the Battle of France.
501 Sq was in France while 145 Sq (SO) was not.
I also feel an O would be more rounded, but the photo angle (including rear damage) can play tricks, we have seen that before....
Not conclusive but my bet is on 501 Sq never-the-less
Cheers
Stig