I must be doing something wrong. No matter what stamp I look at I have yet to see a watermark. For instance the 1c franklin, I have it narrowed down to 331,357,374 which all have watermarks, so I am looking for a SL or DL watermark (likely not a 357 due to stamp color). I wet it with Ronsonol and get nothing. I have tried this on lots of the Washington Franklin stamps but also get nothing. Any hints or tips?
Without a photo or description of your set-up it is hard to troubleshoot. Black tray? Stamp face down? Couple drops of fluid?
Yes, the single-line watermark can be tricky if its position is unusual, but as you note, you are looking at 1 cent green stamps. There should be some that jump out. I would recommend trying a Scott 300 which *must* have a double line watermark and get the equipment/technique working.