1/4-20 x 1-1/8" carriage bolt (4320-01123-18B) with a standard 1/4-20 serrated flange nut (4420-01141-00) on the inside.
You will probably *not* be able to find those bolts; you can substitute a 1/4-20 x 1-1/4" carriage bolt (4320-01123-20B), but you may need to trim a bit off the end.