The Round File

What is the word origin for 'File 13'?
e.g. You're talking about the File 13.

File 13 means the office trash can. "We'll put this document in File 13." I am guessing that "13" is used because it is an unlucky number.

Also called "the round file" because round is a popular shape for trash cans. Even though trash cans can be rectangular, just about the only thing in an office that's round is the trash can.

ESC+ Brian, many thanks for your clear explanations. Is this expresion mainly used in the States or on both sides of the Atlantic?

I am from the New World, so I can't say if either of these expressions (File 13 or Round File) are used in the Old World.