A truck is a nautical term for a wooden ball, disk, or bun-shaped cap at the top of a mast, with holes in it through which flag halyards are passed. Trucks are also used on wooden flagpoles, to prevent them from splitting.
Without a masthead truck, water could easily seep into the circular growth rings of a wooden mast. However, the grain in the truck is perpendicular to that of the mast, allowing the water to run off it.
- Masting and Rigging the Clipper Ship and Ocean Carrier" by Harold A. Underhill ISBN 0-85174-173-8