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<?php

function remove_order_meta_box() { 
	// remove_meta_box( 'woocommerce-order-downloads', 'shop_order', 'normal' ); // Download at shop order page
	remove_meta_box( 'postcustom', 'shop_order', 'normal' ); // Custom field at shop order page
}
add_action( 'add_meta_boxes' , 'remove_order_meta_box', 50 );