Reference Zinc Large 5/8"x 2-5/16"x 5-1/2" ( REF-3 ). 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.