You're only looking at half the oddity. Look again at the description of letters. All caps, but capitals a little larger. I work in printing and see that style of font regularly. It's mostly used in magazines with a snooty kind of attitude. (the New Yorker comes to mind, though I'd have to actually look at an issue to see if I'm right.)
The formatting of the dates is also a magazine-style affectation. This raises the possibility that the person who wrote the letters learned written English by copying a magazine. This doesn't rule out an American. Some of your weird loner types are at least partially self-educated.
Another possibility is that whoever it was used some kind of electronic (probably not online) translator that displays results in a similar format. That's less likely than the above idea, but it should be checked. Hope the FBI is reading this thread!
"You're only looking at half the oddity. Look again at the description of letters. All caps, but capitals a little larger. I work in printing and see that style of font regularly. It's mostly used in magazines with a snooty kind of attitude. (the New Yorker comes to mind, though I'd have to actually look at an issue to see if I'm right.)"