Change the featured image text on custom post type

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add_action( 'admin_head', 'remove_my_meta_boxen' );
function remove_my_meta_boxen() {
    remove_meta_box( 'postimagediv', 'countries', 'side' );
    add_meta_box('postimagediv', __('Country Flag'), 'post_thumbnail_meta_box', 'countries', 'side', 'high');
}

The post type in this example is countries