Does anybody know what this could be? It is wound 1:1, and each winding has 17.9ohms and ~20mH. I've removed the ferrite (It was brittle and reacted to a magnet) and it was a typical E I.
In a different thread (are common and differential mode chokes interchangeable) it was suggested that coupled inductors (pulse transformer) are used for this task.
It's used in a powered bus, as seen in the diagram below. I drew it as a differential choke because that's how I understand it should work. It blocks a superimposed ~20kHz signal from entering the power section.