What is this dark material behind kitchen plaster exterior wall?

Question

While renovating our kitchen the electricians opened part of the exterior wall. It is a plaster wall with metal wire lath in a 1920s-built home, but I can't identify this darker material behind the plaster.

The electricians stopped manipulating the material and asked if I wanted to have it tested for asbestos. The testing is in progress but I'm not sure what this material is to begin with.

It does look fibrous but it is dark. It almost looks like fiberglass, mineral wool, or even cardboard, but it seems to be comprised of strips that are nailed to the studs and/or furring strips.

(The gap in the material was not cut as part of this renovation. A previous owner had bathrooms installed or updated above and below the kitchen; we think the plumbing visible in the pictures is the vent stack and/or sewer line, and the plumbers must have cut this gap in order to install it.)

Answer 1

I had this material tested and it turned out to be cellulose. There was no asbestos detected. I still haven't found the exact product or anything similar to it but it is almost certainly meant as insulation.

2024 update: I found a lot more of this in a bathroom and it appears that it is meant to serve a dual purpose as both lath and insulation. Another room in our house is plastered but the exterior walls have no wood or metal lath, only this cellulose product, and after removing plenty it's clear that its used as lath.

I assume I can't find any product information because it's very old -- I found newspapers in the wall that are dated September/October 1926.

For anyone who encounters this in their home it should be harmless and is much easier to remove than wood or metal lath.