Making watermarks is a complex business and I am certainly not an expert on them. The simplest are just pieces of bent wire sewn down on to the laid or wove cover wire of the mould. These were invented, as far as we know, way back in the 13th century in Italy. However soon after the first practical papermaking machines were introduced in England in the early 19th century, both the dandy roll and the cylinder mould machine made it possible to produce a continuous web of paper with multiple copies of the same water mark. To make all these from wire would have been unnecessarily time-consuming.
The answer was to make one original and to to copy it by electrotyping - a technique also used to make printing plates. In most cases, the wires were soldered to a copy plate. This was pressed into soft wax leaving a reverse copy. graphite was dusted onto the wax but wiped off the surface leaving the carbon only in the impressed lines. Then this could be used to electroplate copper on to the carbon producing a copy of the original that was sewn onto the mould. This is of course a great oversimplification.
About 30 years ago I was lucky enough to purchase a number of engraved plates from the late Ron MacDonald. These had been engraved with the lines required for the watermark and plate is an example. A similar process led to the production of multiple watermarks. The plates are now part of the Hayle Mill Archives though many do not relate to Hayle Mill.