Mo. Rev. Stat. § 515.120
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.
(RSMo 1939 § 1161)
Prior revisions: 1929 § 995; 1919 § 1446; 1909 § 2015