Good point posting that Southern Guard Website.
The problem is that the EOG, Confederate Arms & Equipment book (page 138)is listing some jackets as "Sack Coats", and they are not.
The North Carolina sack coat (page 144)is a true type of sack coat, issued to mostly North Carrolina State troops, early in the war.
Many Confederate soldiers were sent civilian sack coats while off to war, or marched off to war with sack coats from home.
The usual description of a sack coat was a loose fitting, 4-piece body construction, & 2-piece sleeves. The body was usually straight in design, hanging down past the sleeves, usually to below the waist. They usually had 4 buttons, but that was changeable due to tastes in the manufacture of these coats.
There are many variances in the collar design, stand up, and fall down types are encountered. Civilian versions were made out of a variety of material, wool, wool & cotton jean cloth…usually a dark brown to a black color for civilian sack coats. Some coats would have 2-3 pockets, while military versions may possess none. There are many variations of what I described above.
Childs makes one of those jackets that John Stillwagon describes in the Southern Guards website. It makes for some good reading.