Hi Peter!
OK - on the bottom of the reciever - where the barrel joins the cannon assembly - there may be a "groove" that appears like it was struck with "chisel" of sorts, and would "align" both parts together... As in this:
Your's may have a groove - or may not (as in this photo - there is no "witness mark")....
Peter - I can't wait to see any other photos you post... Simply outstanding from what I can see of your HK so far.. Congratulations!!!
John