This is a losmandy rail clamp that was designed to hold a lightweight object on the top rail mounted on a telescope. The round hole in the flat portion of the clamp is for a 1/4-20 screw that is counter-bore (.44" wide X .30" deep) to accept socket-style screws. The horizontal hole through the end of the clamp is designed to accept a 1/4-20 screw with a rectangular hole in the clamp for a 1/4-20 nut.