What kind of building material is this? Who knows this?

The house is from 1969, so you have to be a bit cautious about what might have been installed there...

Our previous house was built in 1953/54 and there we also had such "sauerkraut boards". They contained no asbestos whatsoever. Are you already familiar with the various harmful substances that were installed in the years after the end of the war until the mid-2000s? About three years ago (during the period we sold our house), since potential buyers asked me about it, I researched intensively and came across some helpful websites. Obviously, no links may be mentioned here, but you will surely find something if you look for it. I find it interesting that until 1995 the use of "asbestos cement pressure pipes" was allowed. Asbestos was only gradually banned in small steps. (We are building our house now, in 2022. It is quite possible that in 25 years it will become known that materials were used that will turn out to be harmful after 25 years. This is no criticism or judgment. :) )