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 & formulas: longitudes and anomalies summed up together, no pictures at all.
I am very confused.