According to one of the posts[0] on the photographer's website:

> The metal device visible in the left picture was part of a spinning chair. It held the head in the correct position during the photographing.

[0] http://www.marinamaral.com/blog/2018/4/9/faces-of-auschwitz-...

It looks like just a peg in the wall where people were told to stand to center their head in front of the camera.