This explains it all!

How hard can fake letters be? It’s so low tech that nobody expects it. Photocopying the letter over and over helps make it look more authentic too. By the way, if you use MS-Word and type “111th” it will superscript automatically. If you type “111 th” (with a space) then backspace later, it will not superscript: Just in case you want to do a few fake letters of your own. A great hobby!

One source of typewriter fonts is here.

Here’s a copy of the George Bush signature too.