This tool is used to gently insert a last in a finished shoe to protect the back seam. Used for shoe repair purposes. In Germany it is called "Einleister". New old stock product.
Here you can find a video by Materna on Instagram how it works.
https://www.instagram.com/p/DJ9MLtbtGg1/?utm_source=ig_web_copy_link&igsh=MzRlODBiNWFlZA==
If you have any questions or special needs, please ask.