Cavity Backs: The back of the iron has been 'scooped out'. Like the
770.
Muscle-Cavity: The back of the iron has been 'scooped out', but some material has been left near the center of gravity. Like the
550C.
Muscle Back: Not only is the back of the iron filled in, it's got extra on it. Like the
550M.
Blade: The back is filled in - a solid chunk of metal. Like the
PCF.