Congrats on the 503. In case you don't have the manual you can download one at
http://www.singerco.com/support/instruction-manuals Just put in 503 (not 503a). As you can see from the cam that you have you need what is called "top hat" cams. A good resource for information about the various ones for Singer is
http://www.singersewinginfo.co.uk/pattern_cam_list/
Janey - Neat people never make the exciting discoveries I do.