[paulk]

Drifts are a common fastener for holding large timbers together, and make sense in this application. Chappelle mentions them extensively, as do the Pardeys. Sometimes the ends are peened over if they project, essentially turning them into huge rivets. They are likely what was used originally, and if they''re long enough to hold well, the yard probably saw no reason to change. They are cheaper than bolts, less likely to break in driving than carriage bolts, and because they don''t have threads, don''t have little channels for water (and rot) to get in. A little like metal trunnels, and very traditional. Ask the guys who did the work.