Well, after an hour in the work room, I couldn't find my "muse" and I couldn't find any photos of Panther gun-tanks with the MNH rear box either!
The photo that I had in the back of my mind turned out to be a pic of another Panther with the rear IR equipment stowage box. Aside form "Cuckoo," that was it for tanks.
However, I did find two photos of the box on two different Jagdpanther G1's.
The one book that had both photos was:
Munch. "The Complete Combat History of the schwere Panzerjager Abteilung 654." Published by Fedorowitz.
Page 549, top, has a picture of a G1 from 3rd platoon, 1./sPzJ Abt 654. (You can see the ribs on the box best if you turn the book sideways.)
Page 570, bottom, has a picture of tactical number "123," 2nd platoon, 1./sPzJ Abt 654. This same photo is also published in Panzer Tracts 9-3, pg. 9-3-60 and in the Speilberger & Feist book "Panzerkampfwagen V Panther" (1968) on pg. 43.
Interestingly enough (and totally off-topic!), In the Speilberger-Feist book, that photo is captioned, "A Jadgpanther your author commanded which was knocked out by American anti-tank guns in April 1945." (BTW: This caption contradicts the info in "History of sPzJ Abt 654.")
Both Panzer Tracts 5-3 and 9-3 also have scale drawings of the MNH box on pages 5-163 and 9-3-54, respectively.
So, that makes at least three different vehicles with the MNH box. (Although it's at least arguable that the "Cuckoo" box is a replacement from a Jagdpanther since neither of its rear boxes have Zimmerit, but the tank was clearly manufactured with it.) The MNH boxes existed, even if in very small numbers.
Sorry I couldn't find any other photos, but maybe I can find my model-building "muse" if I look hard enough!
Happy modeling!