Looks like wood fill (putty) to me. Probably a big gouge in the wood and it was filled, then refinished at one point. Just my best guess. I found a lot of the stuff in some beat up shotgun stocks when I stripped them.
Nowadays you can buy the wood fill that comes in shades so it will blend in with the rest of the wood.
My SVT-40 stock has some kind of filler on the stock from the arsenal refurbishing. The filler is hard like an epoxy and they didn't do a very good job smoothing it out.
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You could always strip your stock, chip that stuff out, put the new filler that is close to your colour and re-stain it. Or just go shoot the hell out of it. Marks of use look pretty good on a period piece, stock repairs - not so much.




























