Watermark Question - Oddity Found On GB 1883 2&1/2-D Lilac Sg190

Good evening all,
Today I was going through an old collection I bought recently, and came upon a GB 1883 2 & 1/2 d lilac (SG 190) with an interesting watermark.
The normal watermark for this is W49, Imperial Crown, orientated sideways.
The stamp in question has the correct crown and orientation, but instead of a centrally placed watermark, as usually seen, it has what appears to be a shift, showing two parts of separate crowns. I can't find any note in my catalogues to explain this, and it is the first of this type I have seen. Has anyone either seen another, or have any information that might explain it?
Regards
DavidR

Shifts occur when sheets are fed into the printing press. They become interesting when the sheet margin comes into play. Depending on the stamp, this might result in letters of the word "POSTAGE" or no watermark at all. They also are interesting when they are fed incorrectly (inverted watermark) or gummed on the wrong side (reversed watermark).

Thanks NSK. Not sure how common a watermark shift like this is on early GB though, never having seen one before.
Regards
DavidR

If the watermarks on the sheet weren't symmetrical, the placing the sheet in reversed would shift them all in relation to the stamp, and the sideways watermark would be reversed from the normal sideways one.