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Trimble part number R71204512 a vent for the bottom of the 360° remote target 571 204 310 (prism 571 204 312). A very small amount of gauzing needs to be placed inside of it to impede the path of water. This is a direct replacement for the stock part and lets the prism breathe while slowing the path for water. You will also need the stock O ring which I mic'ed as 5.8mm ID and 2.3 mm high