My bad.
I most certainly should not have used the word "prove" and you are right to point it out.
The letter, a donation to the Sixth Floor Museum, is accompanied with the explanation that this letter was written shortly after the assassination.
The letter doesn't prove he wasn't there, obviously, but it is strong circumstantial evidence that Arnold wasn't there that day and nothing more than that.
I don't see how. For it to be so, then some things have to be assumed without evidence.
- That the letter was indeed written shortly after the assassination
- That Gordon hadn't previously discussed the events of the day with his wife in an earlier letter or phone call
- that not enough time had elapsed for him to have gotten to Seattle before writing the letter
One weird thing is that I know the World's Fair was in NYC in 1963. But I looked it up, and there was a "Century 21 Exposition" (also known as the Seattle World's Fair). The problem is that it took place between April 21 and October 21, 1962.