Get related service resource in browser but 404 when invoked

I've declared a related resource using the services relationships method (using the comments.resource as a basis). The use case is for pulling a summary weather report from a weather site. Both the site and the summary are resources, though a summary only makes sense in the context of a site. The service appears in the browser, but submitting via the services browser gives a 404 Not found. Ditto for calling the service via curl.

A curl call to http://example.com/api/v2/site/100105/summary/2010/01.xml actually invokes the correct handler, passing correct arguments but the service already returns a 404 even before the _weather_services_site_summary_index() method is invoked.

Where should I look to understand why the service definition is sufficient to show the correct data in the services browser but not to actually invoke the service?

function weather_services_service_resource() {
  return array(
    'site' => array(
      '#retrieve' => array(
        '#callback' => '_weather_services_site_retrieve',
        '#file' => array('file' => 'inc', 'module' => 'weather_services'),
        '#args' => array(
          array(
            '#name' => 'number',
            '#optional' => FALSE,
            '#source' => array('path' => 0),
            '#type' => 'int',
            '#description' => t('The site number of the Site to get'),
          ),
        ),
        '#access arguments' => array('view weather reports'),
        '#access arguments append' => TRUE,
      ),

      '#index' => array(
        '#callback' => '_weather_services_site_index',
        '#file' => array('file' => 'inc', 'module' => 'weather_services'),
        '#args' => array(),
        '#access arguments' => array('access content'),
        '#access arguments append' => TRUE,
      ),

      // Map to summary weather reports
      '#relationships' => array('summary' => _weather_services_summary_resource()),
    ), // /site resource

//snipped

function weather_services_summary_resource() {
  return array(
    '#file' => array('file' => 'inc', 'module' => 'weather_services'),
    '#callback' => 'weather_services_site_summary_index',
    '#args' => array(
      array(
        '#name' => 'year',
        '#optional' => TRUE,
        '#source' => array('path' => 2), // in context of site/{site_no}/summary/[year]/[month]
        '#type' => 'int',
        '#description' => 'Year of report',
      ),
      array(
        '#name' => 'month',
        '#optional' => TRUE,
        '#source' => array('path' => 3),
        '#type' => 'int',
        '#description' => 'Month of report',
      ),
    ),
    //'#access callback' => '_comment_resource_node_comments_access',
    '#access arguments' => array('access content'), // @todo: fix - make access weather reports
    '#access arguments append' => TRUE,
  );
}