A short 15mm rod with female 1/2"-13 thread inside and ¼-20 screw hole on the other end.
It is primarily used to retrofit an arm (or similar) that ends in a ¼ 20 screw to a rod end. It can also be used to mount an accessory that attaches with a male ¼ 20 screw. It tightens to your arm or accessory with a small Allen screw.
The 15mm rod with 1/2"-13 thread inside fits Zacuto male rod of any length.