[schmelvin]

The "of the" being smaller doesn't mean he has to be an American. He could have seen it in print--on stationary letterhead if he'd corresponded with his target before and received a response. Or maybe he's seen the street sign itself. Maybe he saw that the "of the" was not capitalized when he looked the address up, possibly getting it off a website.

The third letter is different from the first two in content and the handwriting looks smaller. Were the first two letters published prior to the third being sent out? Could the person who sent the third letter have seen a photo of the others in a newspaper and imitated style and handwriting, then added his 2 cents?

[weegee]

The first 2 letters were IDENTICAL (not just in text, but position of text, exact same differences. Either a photocopy or carbon copy.

There was no text or pictures of the first "2" letters released before the third letter arrived. This means that the similarity of language was not known by the public at large (eliminates the likelihood of a copycat who read about the first "2" letters).