Random Find behind a picture frame!

This seems to be my month of surprising finds! Although not as rare and valuable as my Lincoln Funeral Paper Ribbon found in back of medical book. This is just as fun in my opinion.
Firstly, I love old photographs. Especially old group photographs of the military nature or antique class pictures. I just enjoy the history of them and looking at what people looked/dressed like 100 years ago. I purchased 2 old group pictures framed, one was a group of men of which one was receiving a silver award cup of some nature. The other was a grade school boys and girls graduation photograph. I would say both from the 1920’s or 1930’s.
Anyway, I like to take them apart, when framed and clean the glass of dirt and dust. Hidden behind the class graduation photograph was a WW1 “Son in Service” window poster. After doing some research, I discovered that in WW1 and WW2 the families of those serving overseas would hang these posters in their windows to show support for their loved ones. The Blue Star symbolized how many of their family members were in the war. In this case the one blue star meaning one loved one. On the bottom states printed in Philadelphia, Pennsylvania in 1919. It is a little damaged but really neat, I think!