Hi Greg,
The concept of cast off and other parameters of gun fit were well known. However, of the 10-15 original English and American fowling guns I have examined closely, none showed cast off that I could detect at the time. That said, I remember that the butt stock shaping was not symmetrical on a couple of them. It seemed, if I am remembering correctly, that while the center line did not curve, the left side (when holding the gun to shoot) was flatter and perhaps, hollowed out slightly, compared with the other side.
dave