The feed dogs are usually easier, since they are aligned with the cutouts in the needle plate. I'm pretty sure it's done visually, feed dog movements too. I'm not any help unfortunately, it's just that I have taken an interest in the exterior design of the 503. They are like hen's teeth over here, I have come across a single one, with a mains converter to 120V at the end of the plug, and it was not for sale. I couldn't find any good illustrations in the yahoo group either.