service performed in the employ of a State, or any political subdivision thereof, or in the employ of an Indian tribe, or any instrumentality of any one or more of the foregoing which is wholly owned by one or more States or political subdivisions or Indian tribes; and any service performed in the employ of any instrumentality of one or more States or political subdivisions to the extent that the instrumentality is, with respect to such service, immune under the Constitution of the United States from the tax imposed by section 3301;