Query: "file 13" (throw sth. away)

[Cohen, Gerald Leonard]

    Today a secretary asked me if I know the origin of "file 13" or "file (sth.) in 13,"
meaning to throw sth. away. I had never heard this expression and asked her for an example. She said that if she got sth. of no importance, she might say, "Oh, I'll file this in 13," i.e., toss it in the waste-basket. Years ago I heard a similar expression: "I'll file it under G" (for garbage).

    Has anyone heard of "file 13" or "file it in 13"? And would anyone know why "13" was chosen and not, say, 12?

    Any help would be much appreciated.

Gerald Cohen