They're mounted underneath each engines on the JU-52. I saw them up close at the museum. Each one consists of bundled half-inch pipes running through the whole length, open at both ends.
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The three cylinders hanging below each engine are the oil coolers - this page has a much closer shot which I've reproduced below.
Oil enters and exits the cooler through the two vertical lines and swirls around the hollow air channels. The cold air in flight moving through the tubes cools the oil, which is then returned to the engine.