The story
"Main" refers to the mainmast, whereas a truck on another mast may be called (on the mizzenmast, for example) "mizzen-truck".
Etymology from Wiktionary, CC BY-SA 4.0.
main-truck
Today it 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; also a temporary or emergency place for a lookout. Full definition →