Today I am compiling a list of members of the 13th Missouri Cavalry, MSG, CSA, from the Missouri State Archives, Soldiers' Records. In this data base there are many designations used for this unit. One designation said "6th and 13th Mo." but did not specify whether infantry or cavalry. There is only one entry for this unit designation and it is Col. George B. Spratt, for which the card says:

Spratt George B. (Kansas City) Col.
Surname Given Name Rank
6th & 13th Mo.
Service
Confirmation of Record
Ex-Confederate Missourians Association 9th Annual Reunion
Kansas City, 1891.

It is, of course, possible that there is a transcription error to the card or in the source record.

You should contact the Missouri State Archives to get a confirmation and clarification.

There are three listings for Private George B. Spratt, 6th Infantry, which would make it more likely that this is an error in the recording history of the above record.