https://heim.ifi.uio.no/~trygver/themes/mvc/mvc-index.html[Ed: note the illustration. On a cellphone, so a little painful to read through - I just remember Trygve mentioning it in a presentation I had the good fortune to attend a few years back]
[Ed2: Here's at least one reference to MVC-U, from one of the presentations on DCI (a successor to MVC of sorts):
http://www.artima.com/articles/dci_vision.html
"Most programmers think of MVC as a fancy composition of several instances of the Observer pattern. Most programming environments provide MVC base classes that can be extended to synchronize the state of the Model, the View, and the Controller. (Model, View and Controller are actually roles that can be played by the objects that the user provides—we'll talk more about roles later.) So it's just a housekeeping technique, right? To think of it that way is to take a nerd's perspective. We'll call that perspective "Model-View-Controller." More deeply, the framework exists to separate the representation of information from user interaction. In that capacity we'll call it "Model-View-Controller-User," capturing all four of the important actors at work—MVC-U for short."
As I recall, in the presentation, it was indicated that this was the an emphasis on what MVC originally was about, and that the User-bit originally was such a central part of the whole thing, that it was also part of the name -- but that the "U" got dropped at some point (ie: in the late 70s when they were writing up this stuff initially).]