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The 1939 Harley-Davidson Knucklehead was a significant model, released just before WWII, marking the fourth year of the Knucklehead's production and featuring new elements like the teardrop gas tank and horseshoe oil pan. It was powered by a 61 cubic inch overhead-valve V-twin engine, with the EL model producing 40 horsepower and designed to support a sidecar. Its design pushed the company through the Great Depression, and the 1939 model is particularly valuable due to its limited production num