For some rules-of-thumb on public domain status check:

http://www.public.asu.edu/~dkarjala/publicdomain/SearchC-R.html

The 1923 date comes from the duration of copyright in U.S. law. The term of copyright was 56 years (28+28) in the U.S. from 1909 to about 1962. Starting around 1962 Congress began extending the term. Beginning in 1987 it was 75 years for works published before 1978. In late 1998 Congress extended the term of pre-1978 published works yet again, to a whopping 95 years, but this didn't apply to copyrights that had already lapsed. Hence anything that was published in 1922 or earlier was released into the public domain no later than January 1st, 1998 (1922 + 75 = 1997, but all copyrights are allowed to linger over until the first of the following year). Anything published in 1923 would have had its copyright lapes on January 1, 1999, but promotion of these works to the public domain was stifled by the 1998 law.

T.