Hard to say what the parent material is. Possibly aluminum.
At any rate, I'd use a fine steel wire brush to clean all of the threads taking great care you only brush with the threads. Lubricate with grease after the threads screw in easily. The lids also come with a gasket which cannot be purchased separately. This serves to keep the threads clean and water out of the other side. When making your own gaskets, see here:
Aloha Everyone, I accidentally stepped on and broke the very brittle and yellowed plastic part of my vetus UFO vent on the fore deck of my 1985 Hunter 34. I was able to to find a local Vetus dealer who ordered me a replacement part, SKU VE48. On the Vetus website it showed a picture of VE48...
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