Valentino will look up the details on your specific instrument shortly, but I must tell you the type of walnut you have is known as Grafted Walnut.
Here in California, our native walnut is very good at growing in the clay type of soils we have, but the walnuts are small and not as tasty as the Persian or European walnuts. So the orchards are planted with the native walnut roots grafted to a more delicious walnut for the stem and crown. At the intersection of this graft, very interesting things happen.
The graft in orchards is usually far below the crotch, sometimes almost at the ground, although I have seen the graft as high as the crotch before.