Mo. Rev. Stat. § 515.120
Current with changes from the 2023 Legislative Session
Such notices and all subpoenas for witnesses in any cause may also be served by any person being a competent witness in the cause; and the affidavit of such person shall be received as evidence of the facts stated relating to the service.
§ 515.120, RSMo
RSMo 1939 § 1161