There is a extra detaile on the back of this penny red 1857 stamp , over the watermark which is placed at lower left corner . Could this be marginal watermark or it's just a random shape in the paper ?
Watermarks can shift and there are stamps with a marginal sheet watermark: usually 'POSTAGE.' Yours, however, seems to be a Perkins Bacon stamp, so no.
If the left letter is not A or T and the right A or L (few exceptions), it is unlikely to be a displaced watermark from the sheet margin anyway. As this one has part of another stamp atop, it is extremely unlikely.
It could be the crown watermark has shifted vertically. (Actually, the printing was misaligned vertically).
Thank you ,for the information. I can't see any of these letters here. If I understood the post correctly the detail at the upper section of the stamp should be part of the watermark of the stamp over it. Because the perforation was shifted it apered on the back of the stamp at this place.
It is very difficult to see anything. If this is a stamp without gum, using a black background should show the watermark much better than the current picture.
It looks like this stamp has a crown watermark that is not at the centre of the stamp. Instead, it looks like the top part is on one stamp and the bottom part is on the adjoining stamp below it. So, the sheet was not correctly fed into the press resulting in a shifted watermark (more correctly a shifted printing).