After placing the oversized folio on a large table and then unfolding the even larger Ichnographia, the area of the plan around the Vatican valley circus is the first thing I check.
Within minutes of having a 'real' Ichnographia unfolded in front of me, I noticed that the Circus of Caligula and Nero is not only labeled, but also configured somewhat differently than the circus plan I was used to seeing. Of course, I was instantly very excited because nowhere have I ever read about the Ichnographia having two states/editions (like Piranesi's Carceri/Prisons have two published states/editions), and these two different plans are definitely not noted within Wilton-Ely's recent Giovanni Battista Piranesi--The Complete Etchings. Moreover, I believe I discovered something that no other architect, architectural historian, or art historian had noticed before. I then quickly scanned the rest of the plan to see if any other differences existed, and, sure enough, the Circus Agonalis is likewise different than the plan commonly reproduced.