I'm no expert, but I am just finishing up a halfstock made to look like it had been cut down from a full-stock and modified in other ways over time. The amount of metal work on the halfstock is significantly more than full-stock. As just one trivial example, on a true halfstock, the barrel must be finished neatly on all flats (except perhaps the bottom one) for much of its length, whereas we see many full stocks where the unseen flats are finished less than the visible ones.
Also, I've always been skeptical about the simple "barrel supports the forestock" mantra, esp. when people always talk about extreme swamps where the support from the stock is critical to keep barrel from deflecting. The best way I've come up with it to think about it is that the barrel and forestock being tied together by the pins form a sort of beam structure with significantly more depth than either component alone. With a halfstock, the barrel must be a little beefier, even with a rib.
Ivey has a few halfstocks from NC, one at least that was originally flint -- they were high-end, I think. Wood was always cheaper and easier to work than iron, although that is debatable these days. I also wouldn't be surprised if many of the first generation or two of southern half-stocks weren't used up during the Great Unpleasantness, as they would have been just "new" enough to be considered serviceable, whereas maybe many of the fullstocks were already considered relics and heirlooms or old junk whatever the case was.
