Why did GM opt to use this type of washer when securing the bed planks? What exactly is the purpose and how do they relate to the underside crossmembers? Is an offset countersink and special tool required?
They are not for securing planks, they are the holes for attaching "bed" to the frame. As HB says, the counterbore (not countersink) hole locks the washer from spinning, the square on the bolt fits into square in washer to keep bolt from spinning. Without the counterbore the washer would still spin. Bed wood companies can drill these for you. They are only used at the locations for tying to truck frame. You can buy a counterbore bit and do it yourself if you use the correct technique or a template.
Still doesn't explain why they were designed with an offset.
Sure it does. If the bolt hole was centered, the washer could rotate in the counterbore. With the bolt hole offset, the outer rim of the washer immediately bumps up against the side of the wood counterbore and can't rotate any farther. They have a square hole for the carriage bolts so that the nut on the bottom of the frame can be tightened.
Kevin First car '29 Ford Special Coupe #2 - '29 Ford pickup restored from the ground up. Newest Project - 51 Chevy 3100 work truck. Photos