Quick update on my original request.
I did not get any response to the email I sent to the club and shop.
I did get a response from the Archive in Milan, and talked with Edgardo Michelotti, who has quite a few interesting Stag renderings I have not seen elsewhere. Unfortunately, he wants €350 to send a high res digital image by email.
In the meantime, I found a better quality image elsewhere on the Internet, which was just about good enough for an A4 print on art paper to hang on my office wall.