When printing not bearing a congressional number, except confidential matter, blank forms, and circular letters not of a public character, is done for a department or officer of the Government, or not of a confidential character, is done for use of congressional committees, two copies shall be sent, unless withheld by order of the committee, by the Director of the Government Publishing Office to the Senate and House of Representatives libraries, respectively, and one copy each to the document rooms of the Senate and House of Representatives, for reference; and these copies may not be removed.