The difference between Main-truck and Mainmast
When used as nouns, main-truck means on a wooden mast, a circular disc (or sometimes a rectangle) of wood near or at the top of the mast, usually with holes or sheaves to reeve signal halyards, whereas mainmast means the chief, and tallest mast of a sailing ship that has more than one mast.
check bellow for the other definitions of Main-truck and Mainmast
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Main-truck as a noun (nautical):
On a wooden mast, a circular disc (or sometimes a rectangle) of wood near or at the top of the mast, usually with holes or sheaves to reeve signal halyards; also a temporary or emergency place for a lookout.
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Mainmast as a noun (nautical):
The chief, and tallest mast of a sailing ship that has more than one mast.