Digest 97.01.13 contained a reply from Ed Berlin to S. K. Goodman that
thoroughly confused me.  It dealt with copyright and public domain and
mentioned 1906 as cutoff date.

Up until the 1970's, when Congress began to consider overhaul of
copyright law, the United States law had the same basic provisions since
1891: statutory copyright was granted for a fixed initial term of 28
years from date of publication, renewable in the 28th year for one
additional 28 year term, after which the work entered the public domain.
An unpublished work was covered by common law, but once published, the
work fell under the statutory provisions.

In the 1970's Congress -- knowing that it intended to extend the term of
copyright to something closer to the provisions in effect in most other
countries, and not wanting to deprive copyright holders whose copyright
terms were about to expire and would expire while the new law was being
drafted -- passed a series of interim measures temporarily extending
then-valid copyrights for more than the maximum 56 years.  That muddied
the waters, so that one could no longer assume that once 56 years had
elapsed from publication date, a work was public domain.

The new copyright law was finally enacted in 1976, with an effective date
of January 1, 1978 -- although some of its provisions went into effect
earlier than that.  It is the law now in effect.  FOR THE FIRST TIME the
75-year concept was introduced, replacing the previous 28/28-year
concept.  Copyright for new works is for life-of-author-plus-50-years,
with other provisions for anonymous and corporate works, where there is
no determinable life of the author.  Previously-existing works covered at
the time of enactment of the new law were given the fixed 75-year term
S. K. Goodman mentions.

I hope this eliminates the confusion.

Matthew Caulfield