I have a Zieba folder with skulls on incorporated into the back spacer, but didn't find the memento mori on the inside of the backspacer until I got the folder home. One of these days I'll ask him if there is a back story.
When you cheat death, especially more than once, the basic concept of memento mori tends to mean a little more. As you get older, some of the associated philosophy, such as stoicism, can be tempting as well.