[Flick Lives]

That letter is not typed; it is printed. As a result, it will have microprinting included in the printed page which will identify the exact printer used to produce the letter.

http://en.wikipedia.org/wiki/Printer_steganography

[steve86]

Of course, copying the letter on a monochrome photocopier and sending the copy obliterates that. I did that once on an anonymous letter I sent to the sheriff.

And if the printer was purchased used or with an undocumented chain of ownership the steganography is not useful (unless a search warrant were obtained and the pattern tested for match.