Well I looked closer and its not that exact Lundhal. Its slightly larger and only has 30 windings, which seems much fewer than all their others. I can’t identify the core material - that one I mentioned for tube mics says 10x the THD than normal Mu Metal, so I can’t see why they would use that one. But the construction otherwise looks similar, with the putty. So it seems its a custom transformer for them. Unshielded no less. I don't know enough about transformers to know if there are other brands which look identical to this.
Anyone have thoughts on using transformers in crossovers?