Here is a excerpt from an Excel spreadsheet of a tech sheet that Bob Adler furnished to bajones238 to help him with his '53 Chevy truck. He was kind enough to share it with me. You will see Adler states the bed side angle strip bolt heads are body color. The remaining strips and bolt heads are black except for those with the offset washers which are cad plated.

Bed wood and skid strips black thru 1953, body color 1954
Bed corner strips spot welded to bed sides are body color
Bed bolts: Side: body color. Skid strips: black to 1953. To frame with offset washers: Cd plate.Side to f=rear fenders: body color

I too have speculated about the process that would have used in the production line to result in these finishes.

Mike