Assuming - from the position of the margin - these are rightmost stamps from the earlier picture, this is the normal watermark.
All points at this being part of a sheet used for the production of stamp books, panes QB1 (upright) and QB1a (inverted), The panes of six were catalogued at £35 and £75, repectively, some years ago.
The printer's sheet would have had more than two columns (see below). This looks like the top left of the printer's sheet. My issue with this piece is that it would have been perforated before cutting. Your piece is cut from a sheet. This may be (waste?) material illegally taken from Harrison & Sons' printing works. That would be a reason not to list it.
I suggest you contact Stanley Gibbons, sending them a scan and asking their opinion on what it is. They will not certify it, but they may be interested to list it. If the piece is genuine, it must be quite valuable.
* Catalogue: Stanley Gibbons Specialised, Vol. 2.
From the 'GB Stamp Booklets' websitehttps://www.gbstampbooklets.com/she...erforations/