An employing unit, not otherwise subject to this chapter, which files with the Administrator its written election to become an employer subject hereto for not less than 2 calendar years, becomes, with the written approval of such election by the Administrator, an employer subject hereto to the same extent as all other employers, as of the date stated in such approval, and ceases to be subject hereto as of January 1 of any calendar year subsequent to such 2 calendar years only if at least 30 days before such January 1 it has filed with the Administrator a written notice to that effect.