Holy crap. I just located a copy of an article by Anne Ross called "Esus Et Les Trios <<Grues>>" in Études Celtique 1960. . . Yes, that's "Esus and the Three Cranes." Not only does it contain a photo of the other side of the Esus relief at Trier, but it also includes another set of reliefs and pieces that she thinks are relevant.
Of course, I will have to translate the French before I can figure out what the hell she's talking about, but holy crap. . .
And I was just trying (and failing) to locate a suggested article from Ceisiwr Serith when I stumbled on this!