function ba_admin_posts_filter( $query )
{
    global $pagenow;
    if (isset($_GET['post_type']) && $_GET['post_type'] == 'prodotto') {
        $query->query_vars['meta_key'] = $_GET['ADMIN_FILTER_FIELD_NAME'];
    if (isset($_GET['ADMIN_FILTER_FIELD_VALUE']) && $_GET['ADMIN_FILTER_FIELD_VALUE'] != '')
        $query->query_vars['meta_value'] = $_GET['ADMIN_FILTER_FIELD_VALUE'];
    }
}

add_filter( 'parse_query', 'ba_admin_posts_filter' );

add_action( 'restrict_manage_posts', 'my_search_box' );
 function my_search_box() {
     // only add search box on desired custom post_type listings
     global $typenow;
     if ($typenow == 'prodotto') {
         ?>
         <select name="ADMIN_FILTER_FIELD_NAME">

         <option value="opzioni_info">Filtra per numero legge</option>
         </select> <?php _e('Numero:', 'baapf'); ?><input type="TEXT" name="ADMIN_FILTER_FIELD_VALUE" value="<?php echo $current_v; ?>" />
         <?php
     }
  }