The Rail Mount Adapter is designed to be mounted to 7/8" to 1" boat rails. Works in conjuction with the No. 241 or No. 241L side/deck mounts. The 242 sandwiches together with the side/deck mount to create a rail adapter kit for your railing. It comes with (4) Stainless steel bolts and (2) grub screws that are designed to keep it from rotating while in use. Make sure to find the optimal position for the mount before securing it as tightening the screws will lock it into place and can leave small marks. Fits 7/8” & 1” diameter round rail, and 1” square rail.