In all cases in the State of Missouri where lands have, prior to February 23, 1875, been selected and claimed as swamp and overflowed lands by said State, and the various counties therein, by virtue of any Act of Congress, and said lands have been withheld from market in consequence thereof by the General Government, and the said State and counties have sold said lands to actual settlers, and said settlers have improved the same to the value of $100; said settlers, their heirs, assigns, and legal representatives, who have continued to reside thereon, shall have priority of right to homestead all such lands as may be rejected by the United States as not being in fact swamp and overflowed lands; and it shall be the duty of the Secretary of the Interior to make such rules and regulations as may be necessary to carry into effect the provisions of this section: Provided, That nothing herein contained shall prejudice the rights of any person who may have made actual settlement upon such lands under the preemption or homestead laws prior to February 23, 1875.