"
I don't even know what to say about this one..."
It is a difficult one as I was tying myself in knots just thinking about how it MIGHT look as regards the watermark.
a) I think it is a misplaced watermark
b) I think it is an inverted watermark
c) I think it is also a reversed watermark
All the above needs checking as my brain hurts but the logic goes like this...

Normal back of stamp as you see it
[
HOWEVER... Stanley Gibbons, while listing the inverted/ reversed watermark, does not give a price for it so that may indicate it is not too common

The damage and trimming would obviously affect any value but it might be worth a little effort to check this one out further... assuming it could be inverted and reversed, that is.