In the 1948 manual for the Melchizedek Priesthood Quorums, Joseph Fielding Smith wrote the following:
There are some
revelations still in possession of the Church which were not included. Some of
these we can readily believe were not included because the inspiration of the
Prophet was that it was not necessary, or because some of them had an
application which was not to intended for publication and to be sent to an
unbelieving world. (Joseph Fielding Smith, Church History and Modern
Revelation: Being a Course of Study for the Melchizedek Priesthood Quorums for
the year 1948 Second Series [Salt Lake City: The Deseret News Press, 1947],
20)