Thanks for the additional pics, Xas!
I'd bet on it being a quenching line: It's (unusually) sharp towards the thinner edges while it get's a bit blurry in the thicker middle section. This behaviour would be expected due to different cooling rates. With a high polish and an etchant allowing fine resolution, the crystal structures forming this line might become visible.
BTW, the steel of the pattern welding seems to behave very close to the steel utilized for the edges (central layer). Possibly a steel that responds very well to quenching. (Or the bladesmith perfectly hit the sweet spot for quenching this blade!)
Regards,
Kai
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