Is the "mean orbit" centered on ellipse center or ellipse focus? Mean longitude and mean anomaly of a body are calculated as the angle covered by the body along a fictious circular orbit, and this is ok.
But where is this orbit centered? Five different sources say five different things:
Wikipedia: circle center = ellipse focus

University of Texas: who knows?!? No ellipse at all:

University of California: who knows? Mean anomaly mentioned but not drawn; only eclipse shown, no circle:

Britannica Enciclopedia: who knows? Both circle and ellipse shown, but mean anomaly not shown:

Max-Planck-Institut für Sonnensystemforschung: circle centered on ellipse center

Theory &amp; formulas: longitudes and anomalies summed up together, no pictures at all.
I am very confused.
But where is this orbit centered?

The true anomaly is the angle as measured from the central body between periapsis passage and the object's current location. The orbit is an ellipse with one of the two foci at the central body. This concept is central to Kepler's laws and Newtonian mechanics.
The eccentric anomaly is the angle as measured from the center of the ellipse between periapsis passage and the projection along the minor axis of the point in question onto the minimal bounding circle that encompasses the elliptical orbit. That minimal bounding circle is necessarily centered on the center of the orbit rather than the central body.
Mean anomaly is a fictitious angle. Unlike true anomaly and eccentric anomaly, mean anomaly does not indicate where the object is. Mean anomaly instead is the angular displacement that a fictitious object in a circular orbit about the central body with the same semi-major axis length as the object in question would have passed through in the time since periapsis passage.

Aside: The image from wikipedia is flat-out wrong. The center is correct, but the size is incorrect. The circular orbit should intersect the ellipse, twice. It should be outside of the ellipse at periapsis and inside the ellipse at apoapsis.