At one point during the four-day-bindery-athon the conversation turned to watermarks. The 9# vintage, 100% cotton rag onion skin we were using in the Japanese bindery project is watermarked, so, I took the opportunity to talk about watermarks and paper making with my LSU graphic design student assistants.
This example is from my collection, the engraved monogram is one of my own, the paper is vintage from the same mill as the onion skin we used in the bindery project.
A great website containing a collection of 332 watermarks is:
also, the Robert C. Williams Paper Museum has some interesting information about watermarks and paper in general: