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It seems to be highly refined linseed oil + a staining agent (I don't have the recipe). Look on the Lee Enfield forum, and you will find plenty of info on the original linseed oil based treatment. If...

The good news: It's fixable.

Over here in Mauserland I would get a bottle of Scherell's gunstock oil.
I have no connection with the shop linked below, the link is just exemplary - to save me the...