I've just read The Fitzgeralds and the Kennedys by Doris Kearns Goodwin and discovered an interesting clue to a forgotten custom. In one of the photo sections in the book a group of men and women are shown attending a Fourth of July celebration in Concord Massachusets.
The scene is from the late 19th or early 20th Century and shows a flag being raised. The men in the photograph are bareheaded and holding their hats under their arms. The women are wearing their hats and saluting by holding the brims of their hats. It's the sort of thing people probably wouldn't pass down to their grandchildren and but for the photograph I'd never have guessed the existence of this practice. I'm pretty sure the women aren't holding their hats to prevent them blowing away..the uniformity of the gesture makes it unlikely.
This sort of discovery always delights me. It's like finding a cute seashell on the shore of history. And there are billions more out there to be seen...if we look closely.