Dave Platt wrote:
. . .
The duration of copyright varies, reflecting changes in
the law.
. . .

Just about all you need to know is when Mickey Mouse was copyrighted.
When it was about to expire, the folks at Disney, with the help of
generous campaign donations (known elsewhere as baksheesh) of $6
million, got a nice bill through Congress (the Sonny Bono Copyright Term
Extension Act) to extend the term of protection by 20 years for works
copyrighted after (hee, hee) January 1, 1923. Mickey's copyright of
course began after this date.

Copyright holders who own fewer Congressmen or are unfortunate enough to
have copyrights that predate Mickey will just have to live with shorter
terms. Hey, it's the best government money can buy.

Roy Lewallen, W7EL