Please find attached a scan of the relevant pages. A little secret among stamp collectors is to use Ronsonol brand lighter fluid. From the begin of the 20th century on, these were the only Van Gelder machines for paper with watermarks.
And that's why I can give pretty accurate info to the questions. The "single-line" watermarks used for United States stamps between and can be paper watermarks difficult to find. As the fluid evaporates there is a moment just before the stamp dries when the watermark is most visible.
Currently there is only one book that seems to address, in depth, Van Gelder Zonen and it is written in Dutch. Also, any sources about Van Gelder Zonen would be appreciated.