What is this material under the underlay?

[ModernPerfect]

I'm redecorating the back bedroom of a 1930s house and when taking up the carpet and underlay found this underneath. It's a thin wood sheet layer stained black. It's only a few mm thick but would it likely affect the floor height of the 5mm underlay and laminate I'm going to put on enough to cause clearance headaches with the door?

What is this material? Was it just to help with draft exclusion? Is there any advantage to keeping it or should I take it up?

 

[foxhole]

Coated ply/hardboard ?.To prevent floorboards showing thru carpet .
Could just be stained black by the underlay .

 

[opps]

Looking at on my laptop, it just looks like really old underlay that has compacted over the years.

 

[gary plumb]

could be old linoleum goes brittle

 

[jj4091]

Looking at on my laptop, it just looks like really old underlay that has compacted over the years.

That's what i thought