Purchase order field missing on order emails

-- HikaShop version -- : 2.6.0
-- Joomla version -- : 3.4.5
-- PHP version -- : 5.5.11
-- Browser(s) name and version -- : latest Chrome

When a user selects the Purchase Order option, it is not showing on either the outgoing order email to the user, nor to the designated recipients. Where to look and how to fix? Thanks!

1/ Purchase Order option is a payment method for items purchased.
2/ Published: Yes
Price: 0.00 USD
Percentage: 0
Tax category: None
Order status: created
Order status notification: Yes
Access Level: All

The email we receive only contains the text "Payment method : Purchase Order". It does not include the actual purchase order number.

Where do I change/modify the email content?

Also, when I click on the "Purchase Order" payment method, a drop-down field appears to enter the purchase order number. Not sure how to get this field into the email, nor do I know where the field is "set" or "modified"

This is how I got the PO # to print (the file got overwritten when I upgraded to the latest hikashop version):

Open this file:
htdocs\media\com_hikashop\mail\order_admin_notification.html.php

After this line:
<span style="color:#1c8faf !important;font-weight:bold;">{TXT:PAYMENT_METHOD} :</span> {VAR:PAYMENT}

Add this code:
<br>
<?php $db = JFactory::getDBO();
$db->setQuery('SELECT history_data FROM #__hikashop_history WHERE history_type="purchase order" AND history_order_id='.$data->order_id);
echo $db->loadResult(); ?>

So I looked at the settings in the System->Emails and it is in the HTML area already. And it was there before I upgraded. The upgrade overwrote the PHP files.