The Modern Forgery Hypothesis

[Bluetoes101]

2. The writing has been identified as 17C writing 

What does that mean?

That the style of writing is consistent with the date on the letter.
I'd thought Lisa had commented on this, but maybe I am hallucinating.

Anyway, you don't really need a degree in these things to see. What we would be expecting for the time and place is Italic. You wouldn't expect a 20C hand to write in Italic. 
Though if someone knew of the right paper to use, they probably knew of the right style to imitate too.