The '68 has a maple top the '57 is supposed to be solid mahogany. There are some '57s that are made with a mahogany cap and some were solid without a cap. The way to tell is the cable route in the control cavity. Square route means it was routed before the top was applied. Round means they used a long drill bit to drill from the output jack to the toggle switch and would be a solid one piece (no glued on top) body.