This mod doesn't work  with 1.2.15 because of the following change:

* Moved template tag replacement of pun_include to the top of all replacements to prevent exploitation via XSS vulnerabilities. On top of this, all included files must have one of the file extensions .php, .php4, .php5, .inc, .html, .htm or .txt.

So, if you want to use this mod you could undo this change by cutting the following code

// START SUBST - <pun_include "*">
while (preg_match('#<pun_include "([^/\\\\]*?)\.(php[45]?|inc|html?|txt)">#', $tpl_main, $cur_include))
    if (!file_exists(PUN_ROOT.'include/user/'.$cur_include[1].'.'.$cur_include[2]))
        error('Unable to process user include '.htmlspecialchars($cur_include[0]).' from template main.tpl. There is no such file in folder /include/user/');

    include PUN_ROOT.'include/user/'.$cur_include[1].'.'.$cur_include[2];
    $tpl_temp = ob_get_contents();
    $tpl_main = str_replace($cur_include[0], $tpl_temp, $tpl_main);
// END SUBST - <pun_include "*">

from header.php

and pasting it into footer.php before

// Close the db connection (and free up any result data)

But since this may cause a security issue I would appreciate advice on how to insert

if($post_count == 1) $description = $cur_post['message'];

in viewtopic.php so that $description is available when template tag replacement of pun_include is made.

Any ideas?