In BOTTON BUTTON what did Norma find after she tore open the box unit?

In BOTTON BUTTON what did Norma find after she tore open the box unit?

Explanation

 In "Button, Button" by Richard Matheson, Norma tears open the box unit but finds nothing inside.

There's no clear explanation given for why there's nothing else inside the box.

This lack of content adds to the mysterious and eerie atmosphere of the story.