I asked myself the same question. Many modern made Southern buttplates are very flat. I haven’t had much contact with genuine Southern antique rifles in the flesh, but I’ve seen quite a few really good photographs. I also own a partial set of hardware from a Southern mountain rifle. I can’t say it any better than a metal fabricator friend of mine, who examined the buttplate and said “that obviously wasn’t his first buttplate, it doesn’t have a flat surface anywhere”.
Hungry Horse