Thanks lat9,
OK, understood.
There is one file only with a couple of sets of bof/eof comments: message_stack.php.
One instance has been incorporated into 1.5.6c, but the other has not. Just in case this is still required, I put the diff here again (it if the line in the ADMIN_FLAG condition:
Code:
root@hase:/home/hase/Documents/ZenCart# diff -uw /<download path>/one_page_checkout-master/zc156b/includes/classes/message_stack.php /<local installation>/testshop/includes/classes/message_stack.php 
--- /<download path>/one_page_checkout-master/zc156b/includes/classes/message_stack.php	2020-03-06 23:43:25.000000000 +0900
+++ /<local installation>/testshop/includes/classes/message_stack.php	2019-05-29 22:04:13.560135523 +0900
@@ -3,10 +3,10 @@
  * messageStack Class.
  *
  * @package classes
- * @copyright Copyright 2003-2016 Zen Cart Development Team
+ * @copyright Copyright 2003-2019 Zen Cart Development Team
  * @copyright Portions Copyright 2003 osCommerce
  * @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
- * @version $Id: Author: DrByte  Sat Oct 17 22:52:38 2015 -0400 Modified in v1.5.5 $
+ * @version $Id: mc12345678 2019 Apr 30 Modified in v1.5.6b $
  */
 if (!defined('IS_ADMIN_FLAG')) {
   die('Illegal Access');
@@ -29,9 +29,6 @@
       for ($i=0, $n=sizeof($messageToStack); $i<$n; $i++) {
         $this->add($messageToStack[$i]['class'], $messageToStack[$i]['text'], $messageToStack[$i]['type']);
       }
-/*-bof-checkout1-lat9  *** 1 of 2 *** Move to output function to allow messages to survive after header redirects
-      $_SESSION['messageToStack']= '';
-  -eof-checkout1-lat9  *** 1 of 2 *** */
     }
 
   }
@@ -62,7 +59,7 @@
 
   function add_session($class, $message, $type = 'error') {
 
-    if (!$_SESSION['messageToStack']) {
+    if (empty($_SESSION['messageToStack'])) {
       $messageToStack = array();
     } else {
       $messageToStack = $_SESSION['messageToStack'];
@@ -80,9 +77,7 @@
   function output($class) {
     global $template, $current_page_base;
     
-//-bof-checkout1-lat9  *** 2 of 2 *** Once the messages are output, it's safe to clear any session-based messages
     $_SESSION['messageToStack'] = '';
-//-eof-checkout1-lat9  *** 2 of 2 ***
 
     $output = array();
     for ($i=0, $n=sizeof($this->messages); $i<$n; $i++) {
@@ -110,4 +105,3 @@
   }
   
 }
-?>