I agree that looking at an actual stamp can be quite helpful for watermarks.

Back of a Sweden 1911-19 Scott 95 1c black
watermark 181- Wavy LinesThis is with watermarking fluid. I did not see the watermark in this case without it.
Note two slightly curved lines at the lower left and upper right of the stamp? These lines obviously are fairly far apart. That should help I would think.
