Here's a first day cover I picked up this year. It's a cool little piece of history.
The American War Mothers organization was formed in 1917. Here's a bit of their history.
When the envelope arrived I was surprised to see that there was something still sealed inside after 84 years. I carefully cut one edge.
This is what I found:
Here's the stamp.
Most of you will have recognized the image as a copy of "Arrangement in Grey and Black No. 1", popularly known as "Whistler's Mother".