found at DigiDirect
The Manfrotto 131DDB Tripod Accessory Arm for Four Heads (Black) mounts onto the centre column of tripods with 3/8 inch threads. The platforms feature 3/8 inch screws, which allow a standard tripod head to be mounted on either or both sides. In addition, the horizontal bar features 3/8 inch screws on both ends, allowing mounting of tripod heads from the sides. Replaces part number 3153B.