Many forged documents can only be shown to be forgeries by 'proving a negative'. There are few, like these ones, that can be positively shown to be forgeries.
Suppose that somebody had a photo of Einstein standing in front of a blackboard with a bunch of seemingly-random numbers on it. Would the photo be plausible? Perhaps. Suppose a check of the Illinois Lottery website showed that the numbers on the board were drawn on 1-1-2004. Would you still consider the photo plausible?
Given that the positive proof of fakery exists, the notion of 'typewriter searching' is silly.
I totally disagree. You are usually not proving a negative when proving a forgery. You are proving a positive--that the document was forged. Here, they want us to have to prove it was impossible to make that document in 1972. That means the burden of proof is not only on us, but that they can defeat assertions of forgery by merely producing a typewriter that can do the various things that appear in the document. That proves nothing, but they get the benefit of not having to admit they are forgeries if that is the way it is played. That is intolerable, in my book.