wartime Russian wood finish

[Schnitzelmahn]

yes, can confirm that is the BC mark in on my stock

so any theories on what the dark spots in my stock are?

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[WarPig1976]

so any theories on what the dark spots in my stock are?

It's called mineral streaking. Your stock is made from Baltic Birch wood 'a.k.a' Russian Birch 'a.k.a.' Arctic Birch.
There's some oil stains mixed in too and probably some remains of the older finish that wasn't completely sanded off.