Mo. Rev. Stat. § 301.320

No person shall operate a motor vehicle or trailer on which there is displayed on the front or rear thereof any other plate, tag or placard bearing any number except the plate furnished by the director of revenue or the placard herein authorized, and the official license tag of any municipality of this state, nor shall there be displayed on any motor vehicle or trailer a placard, sign or tag bearing the words "license lost", "license applied for", or words of similar import, as a substitute for such number plates or such placard.