While renovating a bathroom in a quite old house in germany, we removed the cover holding the tiles on the front side of the bathtub and could not clearly identify the material. It seems to be interwoven strands of wood(?) with a dark paint covering them.
One person was afraid it might be asbestos paint - If someone here knows what it is, please help us satiate our curiosity and hopefully put that worry at rest.
Here's some pictures, tell me in the comments if I can provide anything else to help!