Bill's reason for the name beltline sounds good. I called it a ribbon once on this site and was quickly corrected.
I had always thought that the name "belt" was because someone thought that it looked like a belt.
On Advance-Design (47-55) trucks, the cream medium striping color was also used with several other exterior colors (4 or 5 different striping colors were used, depending on the exterior color).
The pickup cab's top was atttached to the cab bottom at the top of the beltline, but that was not the case for panel body trucks. On those trucks the roof was attached (to the lower body) above the windows.
Tim
