Mo. Rev. Stat. § 341.040
That there shall be in every city of fifteen thousand inhabitants or more, a board of examiners of plumbers, consisting of three members, one of which shall be the chairman of the board of health of said city, who shall be ex officio chairman of said board of examiners; a second member, who shall be a master plumber; a third member shall be a journeyman plumber. Said second and third members shall be appointed by the mayor and approved by the council or city commissioners, as the case may be, of said cities within three months after the passage of sections 341.010 to 341.080, for a term of one year, and shall be paid from the treasury of said city, the same as other officers, in such sums as the authorities may designate; provided, that in no event shall such sums paid second and third members exceed the amount paid for certificates, as provided for in section 341.050; and further provided, that the second and third members of said board shall have at least six years' practical experience as plumbers and be actually engaged in or working at the plumbing business at the time of such appointment.
§ 341.040, RSMo