Joe: the problem with copyright Dates is that the composer's copyright (if composer is known) lasts for 70 years from his death. The sheet music publisher's copyright (assuming he has obtained permission from the composer, if the music is still copyright)lasts for 25 years from publication (and the depositing of necessary copies).
Thus if a composer has been dead for 70+ years, the tune is fair game, but, you still can't photocopy the sheet music if it was produced within the last 25 years. The publisher has a valid copyright on the layout.
If you wish to photocopy vast swathes of sheet music (e.g. Messiah) for a paying public performance, the composer will not object, But make sure you photcopy it from a copy with a copyright date pre 1977.
Hope this clarifies.

Nigel