Reference Zinc Medium 5/8"x 2-1/8"x 4"( REF-2 ) 1/2" through hole with 1" counter bore. Designed for Electro Guard Corrosion Control Systems. Suitable for other flat areas, including: Trim Tabs, Rudder and Struts where round Rudder/Trim Tab anodes may be unsuitable. Hydraulic Swim Platforms. The anodes rectangular shape fits the angle bar well.