Here is the Semi-Postal series of 1943
According to Scott, the Multiple NZ and Star, when viewed from the back, should look like this:
As can be seen, this is the reversed NZ above the Star. In a recent purchase, I seem to have one that is an inverted watermark. As viewed from the back, we have the normal one on the right, and my example on the left:
There are numerous listing of inverted watermarks in Stanley Gibbons, but not for this stamp. SG calls this watermark 98, and has some discussion about watermark sideways in that description.
Of course, to complicate things, this is a triangular stamp, which makes me think that half of the stamps were watermarked normal, and half were watermarked sideways due to placement on the sheet. SG just forgot to list this anomaly. Agree?