I agree with Haze. The more I look at it the more I realize how the knife would be different without it. Assuming the machine that cuts this can only move in a true arc, and cannot transition during the cut (IE cut a parabola), the arc would have to be a bigger radius to meet the lockbar access cutout and keep the existing handle thickness. That would make the knife flatter on the bottom, due to the bigger arc, and you might lose a bit of that nice curve towards the but-end. The other option would be to keep the arc the same radius, but reposition it to have it cut further into the handle before swinging around to the lockbar access cut out. This would make for a thinner handle, and change the look.
I think what we are seeing is a compromise in design to achieve the great proportion and ergos of the 21, based on what CRK's technology could do at the time the 21 was introduced. The latest machinery could likely do something more seemless and fancy, but if it ain't broke.........