-- Last Updated: Nov-05-12 12:20 PM EST --
It's 10-24, ie size 10, 24 threads per inch, Flathead SS machine screws you want, along with four locknuts and finish washers per seat, two ea per thwart, preferably four on the carry yoke. MacMaster Carr has a fine selection including 6" lengths. I have no explanation for the Phillips drive; why not in square, or better yet, hex or torque drive? E mails to Robertson, the square drive folk, remain unanswered as to longer lengths.
McMaster Carr also has high strength blind rivets and SS finish washers, so one-stop shopping there for you.