All salt wood stocks were Claro walnut, like yours, though not all Claro walnut stocks caused rusting/corrosion problems.
And that is a terrible finish, it looks like spar varnish. The original finish on those rifles was prone to cracking and peeling, so someone may have slapped a coat of spar varnish on it to cure the looks.
BTW, that rifle was made in 1969, in case you were wondering. That was right at the peak of the salt wood problem.