The Modern Forgery Hypothesis

[ReneZ]

But also, in one's acceptance or rejection of the probability of this or any evidence, it is very important- required, really- to make the calculation by including the entire picture.

You may do that, but I was talking about the watermark. By itself it adds nothing.
If you are already suspicious about the letter, you may find every other aspect suspicious.

Still, the situation with the watermark of the Beinecke letter is completely staightforward. It is one that was used in the 17th century.