Mo. Rev. Stat. § 700.055: Serial numbers required when, form.

From and after January 1, 1974, the manufacturer shall cause a serial number to be stamped on the front cross member of the left-hand side so that it may be easily read. It may not contain more than fifteen digits. Any multiple units shall contain the same serial number with letters of the alphabet designating that each is a different separate unit. Starting with "A" each additional unit shall be in alphabetical order. The letters shall be stamped at the end of the numbers.