Zacuto Z-RE-FF3 3" female rod extension has been discontinued, here are some related products you might like:
3-inch female/female rod extension. Female on both ends. 1 rod extension only.
Zacuto's rods are all lightweight; 15mm diameter with 60mm spacing and clear anodizing. They are made out of 6061 aluminum and the threads are ½” 13.
More information about Zacuto Products (Link to Zacuto Website)