ᐅ What kind of building material is this? Does anyone recognize it?

The niche where a radiator used to be is being removed so that the window above can be turned into a door.
Under the plaster, I found what is shown in the attached pictures.
What exactly is this? Should I be concerned when working with it? Behind this strange "insulation" (?) the wall is made of bricks coated with a black layer.
The house dates from 1969, so I am cautious about what materials might have been used back then...
Regards
N

The grey material on the left is a wood wool lightweight board, also known as a sauerkraut board and other names it might have.

Then there is probably plaster. I can’t say anything about the black material next to it.

The black material could be bitumen. But it might also be something completely different. Perhaps a professional should take a look.

It is very likely that asbestos fibers are embedded in the insulation material or panels.
Indicators: year of construction, heating niche, gray color, fibers
This can be tested in an independent laboratory. The cost is approximately 100€ (about 110 USD).

ypg schrieb:

It is very likely that asbestos fibers are incorporated in the insulation material or boards.
Indicators: year of construction, heating niche, gray color, fibers.
This can be tested at an independent laboratory. The cost is around 100€ (about $110).

If @ypg is correct, an FFP2 mask is NOT sufficient to safely handle the removal without risk! So please be careful!

Under the keyword sauerkraut boards, I found this:
Sauerkraut boards or Heraklith boards may look like they contain asbestos at first glance due to their gray color, but you don’t need to worry at all, even with older boards: sauerkraut boards generally do not contain asbestos.