The ad island is referenced as $island_insert in the scripts. I believe the code that drives this is in /libs/ubbthreads.inc.php; however, I've run a grep of the front end files, on a stock install, for the island reference that you can dredge through below.

Code
[root@server ~]# grep -r "\$island_insert" /public_html/forum/
/public_html/forum/libs/ubbthreads.inc.php:    $island_insert = "";
/public_html/forum/libs/ubbthreads.inc.php:    $island_insert = ob_get_contents();
/public_html/forum/libs/ubbthreads.inc.php:    return $island_insert;
/public_html/forum/libs/ubbthreads.inc.php:    $island_insert = '';
/public_html/forum/libs/ubbthreads.inc.php:    $island_insert .= $body;
/public_html/forum/libs/ubbthreads.inc.php:    return $island_insert;
/public_html/forum/templates/default/post_top.tpl:{if $smarty.section.post.iteration == 1}{$island_insert}{/if}
/public_html/forum/templates/default/post_side.tpl:{if $smarty.section.post.iteration == 1}{$island_insert}{/if}
/public_html/forum/scripts/showflat.inc.php:           $island_insert = get_portal_box("portal_box_{$forum_info['FORUM_ISLAND_INSERT']}.php");
/public_html/forum/scripts/showflat.inc.php:           "island_insert" => & $island_insert,

Please note, however; modifications to base code are unsupported, and you may be asked to roll back any changes in order to troubleshoot your forums in the future.

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