first guess would be water
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I think it’s a shut off valve
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The water hit different in California
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It appears circles and triangles are chosen because the lids cannot fall into the holes. Some water covers actually have a valve knob underneath, and the lid flips up. Like a sprinkler or pipe valve valve nobhttps://www.envirodesignproducts.com/blogs/news/95979142-do-manhole-covers-have-to-be-round …
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Triangle is an alchemy sign for water. So the shape may communicate what is under alchemist-fool-proof way. https://cs.m.wikipedia.org/wiki/Soubor:Alchemy_water_symbol.svg …
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Constant width => lid can't fall in hole
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Toblerone mine.
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