The Modern Forgery Hypothesis

[ReneZ]

The situation with the watermark and its relevance for the genuine/fake question of the Marci letter is really straightforward.

Let's look at an extreme, hypothetical situation.
In this situation, the watermarks of all Marci's letters would be known and would all be the same, and genuine 17th century. Now the watermark of the Beinecke letter would be different, yet also genuine 17th century. 
What would be the conclusion?
It would be: "Aha, he used different paper for this letter".

Now let's look at another hypothetical situation.
In this case, we know the watermarks of many or all of Marci's letters, and there are several (genuine) different ones.
This shows he used different types of paper. That the Beinecke letter is also different is completely normal.

In reality, we do not know the watermarks of any of his other letters, with the exception of perhaps one that was drawn out in this thread.
So, in reality, there is absolutely nothing to be concluded from the watermark.