Mo. Rev. Stat. § 453.150

Any person adopted by deed of adoption or agreement of adoption in writing prior to 1917 and wherein said instrument was filed for record prior to July 1, 1917, shall hereafter be deemed and held to be for every purpose the child of its parent or parents by adoption as fully as though born to them in lawful wedlock, and such adoption shall have the same force and effect as an adoption under the provisions of this chapter, including all inheritance rights.