Works published in 1923 in which copyright was renewed are still under copyright in the U.S. The last "free" year is 1922. Joe's words "1923 or earlier" are a misprint for "1922 or earlier".

"Common law copyright" is falsely so-called according to some scholars. The more precise term, in this view, is "common law right of first publication". These common-law rights are now preempted in the U.S., for post-1977 works, by statutory rights, but they still exist in pre-1978 works. Some of the oldest such rights will begin to expire on January 1st, 2003. These old rights are relevant to folk music because even if a folk song was published in 1917, say, someone with the right geographic credentials can claim that the published version was his grandpappy's variant of the song, that his grandpappy never "authorized" the publication of the variant, and that he inherited the common-law R.F.P. If the claim is accepted, it makes the grandpappy's supposed variations pre-1978 unpublished matter, rather than published matter from 1917.

A false claim of this sort was raised on the words of "The Wreck of the Old 97". The court case was called George v. Victor Talking Machine Corp. The claim was accepted by the district court and rejected by the court of appeals for the 3rd circuit. Due to a technical mistake on the part of Victor's counsel, however, the litigation dragged on for several more years before the false claim of authorship was finally put to rest. Citations can be found in endnote 22 of this essay.

The current term for author copyrights in the EU is officially life+70 years (not life+75). But some French courts have recently added additional time to the life+70, due to some wording in the French law that was intended to be harmonized by the life+70 provision!

Around 1995 the U.S. copyright in some foreign works (such as Prokofyev's Peter and the Wolf) that had previously been PD in the U.S. was restored by a law called the URAA.

Some information that can assist you in determining the copyright status of old music is here, here, and in Stephen Fishman's book The Public Domain.