Here’s a tidbit. I dug through my backlog of Infocom data, trying to find anything more about Cornerstone. All I could find was this quote:
Currently, we have three types of interpreters: ZIP (for Zork Interpretive Program) which runs the games, MME (for Mu Machine Emulator) which runs Cornerstone, and DIP (for Display Interpretive Program) which runs Fooblitzky.
I think the term “Mu Machine” has gone unnoticed until now. (“DIP” is mentioned here.)