Richard Holmes' book "Redcoat" pp. 121-124 has a well-documented
discussion of the rank of Sergeant Major.
Later in the book he claims that the rank of Regimental Sergeant Major
was introduced shortly before the First World War. Prior to that
"Battalion Sergeant Major" was common.
From a 1766 Military Manual he quotes - "the choice of a sergeant
major must never be influenced by any consideration save that of