91/30 Stock Bedding

[Singer B]

I recently acquired a 1936 non-matching Izhevsk 91/30. The stock is an un-refurbished pre-WWII version. It was very filthy but was not shellacked and the metal of the rifle had original patina so it was obvious that this rifle did not experience the post WWII refurbishment process. Once I removed the barrel and action from the stock, I saw that it appeared as if the interior of the stock had been coated with something. Whoever did it, they were pretty sloppy. Can anyone tell me what this stuff is and should I leave it as is? The action seems to fit well but the barrel has a lot of wiggle room at the end. Thank you for you help!