I think it's some sort of glaze... Definitely not wood, or tar.....
Well stuck on though and I'm pretty sure plaster wouldn't stick to it.... Maybe blue grit would?
I'm sorry I have no idea what it is, but it is similar to something I've found in my Victorian terrace.
One of the earliest layers I've dug down to was on my stairway - the whole wall was covered in a dour painted false woodgrain effect.
It must have been depressingly dark walking up those stairs in mid-winter!
Luckily for me, it came off in sheets, as it hadn't adhered well to the wall.
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