attwood SP-67904 Swivl-Eze Lock'N-Pin Aluminum 1.77 Series Snap-Lock 9-Inch Boat Seat Round Base Plate Hard anodized, corrosion resistant aluminum seat base plate 9 inch round seat base plate delivers a great combination of value and performance Nine inch round base plate requires 2.5 diameter mounting Hole Seat mount designed to permanently lock into place in any swiveling position