Manfrotto 131DDB Accessory arm for 4 heads -BlackManfrotto 131DDB Overview The Manfrotto 131DDB Tripod Accessory Arm for Four Heads mounts onto the center column of tripods with 3 8" threads. The platforms feature 3 8" screws, which allow a standard tripod head to be mounted on either or both sides. In addition, the horizontal bar features 3 8" screws on both ends, allowing mounting of tripod heads from the sides. Replaces part number 3153B.