country_id declared as serial; isn't

uc_store_schema contains the following declaration:

  $schema['uc_countries'] = array(
    'fields' => array(
      'country_id' => array(
        'type' => 'serial',
        'unsigned' => TRUE,
        'not null' => TRUE,
      ),
      ...
  );

The 'serial' type specifically refers to auto_increment fields. I guess that at some point in the past this was an arbitrary ID, but since then it has been repurposed to reference ISO-3166 numeric IDs.

This distinction doesn't make much difference at the moment, since the official Ubercart policy on country def files is still to INSERT the data directly using a db_query() call, but if the country system is ever to be updated to use the drupal_write_record() function it's a change that will have to be made.

TBH the whole country system should be updated so it's less of a manual insert process and more of a "declare values to be imported" thing - not unlike a hook_schema implementation - but that's a matter for another time.