Where do you put the file location in a business letter?

If the file location is the subject of the letter or of some other significance to the content of the letter, it should be part of the 'reference line', just under the inside address, just above the salutation.

If the file location is a code for the sender's own files, the code belongs at the bottom left, several spaces below the typed name of the sender, the 'copy' notation, or the 'enclosures' notation, whichever is last.