There sure aren't very many good images of stitch watermarks on the internet. Here's one of the few I've found which is more-or-less clear - I've nabbed it from the DGL Philatelics page on Nova Scotian 10 cent stamps. As with most watermarks, sometimes they're almost impossible to see without some sort of detection aid, but sometimes you can see them fairly easily.
The Canadian Stamp News has an article which goes into some detail about how stitch watermarks are formed. Have a look at that - it explains how the watermark is essentially accidental in nature. They've been found on Canadian stamps right up to the KGV Admiral era, so certainly any German States issues would have the potential for stitch watermarks, I would think.
if anyone has one of the watermarked ones, please, please show me what they look like - I'm yet to find any I've heard it said that all the genuine ones are signed by now, but it seems that's just not true
1. I would be concerned with any German "signed stamps". Look up Blum Case - a dealer who faked 650 postmarks & 50 expert signatures on metal stamps. It changed a lot in the world of German expertization. 2. The stitch watermark that I am familiar with is not a true watermark and VERY random. It appears on several countries and when expertized adds much value but I am not sure exactly why it should
As far as I know it is from the paper making - the wet paper passes on a mesh and the mesh sections might be "stitched" together leaving an impression in the paper It may appear anywhere and might be vertical or horizontal