By Colleen Howe BEIJING, Oct 20 (Reuters) – China's imports of Mongolian coal reached a record monthly high in September, customs data showed on Monday, as Beijing's efforts to tackle overcapacity pushed up domestic prices and sent traders looking for cheaper supply. Mongolia shipped 9.29 million metric tons of coal to China in September, according to China's General Administration of Customs. That marked a 33% surge from a year earlier and the highest monthly level on record since customs started reporting the data in 2015. China has placed restrictions on coal production in recent months following an unexpected supply increase in the first half of the year that weighed on prices, widely perceived to be part of China's so-called "anti-involution" campaign aimed at curbing overcapacity and

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