Those bolts used on flat car decks, with a square shank below the head (similar to a carriage bolt), but with a flat and countersunk head are actually called "Plow Bolts." Installed, there is no (rounded) head protruding above, and used (obviously) on historic farmers' plow with wooden structures.The flush heads would not interfere with other fittings or tackle on the plow.
Specialty hardware suppliers still inventory Plow Bolts.
