Dear Kelly,
I used them when there were not any others. And several years ago, I found a treasure trove of such patterns in a costume shop that was being thoroughly cleaned for the first time in many, many years. I had my students choose one and make it up for our fashion show, which had a vintage design section this year. It was the highlight of the show.
Three holes in a triangle means this edge should go on a fold; two large holes near the top center and bottom center of the pattern piece means that they should be aligned along the straight grain of the fabric.
I can't remember them, but if you'd tell me what symbols you don't understand, I may be able to help you. It might help to find some contemporary sewing books at your library or online. These patterns were used from the late nineteenth century through the 1950s.
Teri