Unusual material in interior wall

[Colino]

Hello!

I'm new to the forum and completely unknowledgeable — therefore, I appeal for your wisdom.

I live in a condo apartment from '65 and have recently repainted the apartment. I also took the opportunity to replace the baseboards and noticed, to me, an odd material in one of the interior walls. On the inside of the wall, there is a radiator and a window above. On the opposite side, there is a balcony.

Do you know what this material is?

 

[Colino]

Hello!

I am new to the forum and completely ignorant — I am therefore appealing for your knowledge.

I live in a housing cooperative apartment from '65 and have recently repainted the apartment. I also took the opportunity to replace the baseboards and noticed an unusual material in one of the interior walls. On the inside of the wall, there's a radiator and a window above. On the opposite side, there's a balcony.

Do you know what this material is?

After intensive googling, I have now concluded that it is probably straw or flax used as an insulation material. If anyone has other input, it is welcome.