As I recall K's were hard tack (cracker), little can of potted meat to spread onto the cracker, a bullion cube to make broth (soup?), packet of instant coffee & hot chocolate, chewing gum, candy (often M&M's) and a 4-pak of cigarettes.
"C" rations (combat rations) were more complete in the cracker department, and here the "wet" part of the ration would be something like beef stew, beans & franks, sausage patties (God forbid), corned beef hash (double God forbid), ~ with the newer C's (into the Korean conflict) the wets got better ... like beef steak & potatoes! There would also be a can of applesauce, fruit cocktail or peaches. The left over WWII C's would have a full pack of cigarettes whereas the later Korean era (my time) had only the 4 cigs to a pack.
Where Spam came into the picture was when there was a field mess tent set up, or, hell, even the mess hall while in garrison - and Spam and eggs were a common breakfast (when not serving S. on the Shingle!
Quite often on my camping/fishing outings I've carried the smaller tin of Spam as it's no problem with refrigeration. But about one breakfast of spam & eggs is enough to last for awhile. Let me add here, however, (it comes on me like a fever) .... every so often, I just have to have some S.o.S ~ now, true S.o.S. (S on the shingle), is hamburger & gravy on toast ... NOT chipped beef & gravy on toast.
Now that this has all been posted .... maybe I'd better get another small tin of Spam next trip to the super market!