Hi, thanks to Krikor I have solved my issue,
here is the code that I have added to "item.php" file inside my K2 sub-template folder:
<FORM class="yourclass"><INPUT Type="button" VALUE="<?php echo JText::_( 'Back' );?>" onClick="history.go(-1);return true;"></FORM>
Then I added the language override to translate the 'Back' string.
It works for me on joomla 3.6.5 and K2 2.7.1
Thanks Krikor for the precise hints!
Best
Marco