The D-Nut Assembly is the hardware necessary to bolt the tower to the board on any Sky Ski board built in 2009 & prior. The assembly consists of: (1) 5/16" x 18 Aluminum D-nut; (1) 5/16" Countersunk Stringer Washer; (1) 5/16" x 18 x 1" SS FHCS bolt. Four of these hardware assemblies are required to bolt the seat tower to the board.