The proof necessary to establish the citizenship of applicants for mining patents must be made in the following manner: In case of an incorporated company, a certified copy of its charter or certificate of incorporation must be filed. In case of an association of persons unincorporated, the statement of their duly authorized agent, made upon his own knowledge or upon information and belief, setting forth the residence of each person forming such association, must be submitted. This statement must be accompanied by a power of attorney from the parties forming such association, authorizing the person who makes the citizenship showing to act for them in the matter of their application of patent.