This is the 55/56 upper grill bar (bar is smooth).
This is the 57 upper grill bar (note the indentations for the grill to bolt to).
This is the 55/56 lower grill bar(note tabs that the grill bolts to).
This is the 57 lower grill bar (there are holes in the bar that the grill bolts to).
Note: I got these pics from the brothers site.
http://www.brotherstrucks.com/
Their pictures for the lower bars are transposed. Ask me how I know this.
Kim