meadow
Deeply Rooted
They (Burpee's) were using the term saddleback in conjunction with having a deep crease, and comparisons with another bean they carried named White Creaseback. Somewhere in there they used the term "Fat Horse" which made me think of a (fat) horse's back, and the way it looks along the spine.I always thought saddleback refered to the shape of the seed. Bent more at the eye so the bean sort of formed a saddle when viewed from the side.
Wait a sec! I'm certain that I saw something more definitive...
1901: Old Homestead, or "Kentucky Wonder." This most popular variety is an improved large-podded strain of the Southern Prolific. The pods, usually seven inches or more in length, are so fleshy that they are greater in width than breadth, being deeply creased or "saddle-backed." They are solidly meaty, entirely stringless, and of finest quality. If kept gathered as they mature, the vines will continue to bear to the end of the season. We have sufficient stock now to fill all retail orders for this popular bean.