I did a little google research on this and found a site that reprinted recipes from the Army cookbook during World War II. Interestingly enough, there were 4 recipes for this item, 2 of which I never heard of, and the other 2 used either dried chipped beef in a white cream sauce served over toast OR hamburger ground beef in a brown beef gravy served over mashed potatoes.
What they made depended on 2 things - the supplies the Army sent to the Quartermaster Corps (cooks) and/or the cook's choice.
I guess that explains why some vets swear that SOS is chipped beef and others swear it is hamburger. They are both right!
One item I found interesting - one vet wrote in. He was stationed on a Pacific Island, tropical heat and miserable. They had no cold/frozen foods because they did not have the facilities to store them. So they did not have hamburger meat to use. The food they worked with were all canned, boxed or dried. They had dried chipped beef in cream sauce (probably made with dried milk).
So I guess there is no right or wrong about what SOS is. Fortunately for me, we like it both ways - we just call them by different names.