I've only had the extreme pleasure of handling one original Hawken, but I've studied them pictorially for a long time. The forearm of a Hawken rifle is never slab-sided. Nor is any other muzzle loading rifle that has proper architecture. Even though they are deeper vertically than rifles we're more used to seeing, because of the 7/16" to 1/2" rod and the ~ 1/4" web between barrel and rod hole, a result of an under-rib of that dimension, the bottom of the forearm is rounded nicely and never flat. The sides are a gentle convex curve up past the escutcheons and then a quicker convex curve to the barrel channel. Also adding to this is the fact that the bolster on the lock plate is sometimes as thin as ~ 1/4". Once the lock panel forward of the lock is defined, that further influences the shape of the forearm. But they are definitely not slab-sided or flat. I don't know what you mean by "beavertailed".