You present a very interesting question, John. I have wondered for years whether the sound of the Koetsu's ( as an example) differ only due to the type of cartridge body used. Does anybody actually know if the internals ( motor structure, etc.,) of say again, the Koetsu's, actually truly differ from cart to cart as one goes up the line?
With the cartridge that I am currently using, the Benz Ruby, the sound does seem to be influenced by the use of the wooden body. I have heard other Benz cartridges with very similar motors to the Ruby and with different bodies that do have different ( albeit very slightly) sounds. As to the reason, I suspect that the damping factor of the body material has a lot to do with the overall sound of the cartridge.