Originally Posted by Noam Zane
I'm marvelling that someone that can have a stable of bikes and claim to love wrenching yet is stymied by a common, conventionally threaded headset design.
The headset in question isn't "conventionally threaded". I've worked on a lot of conventional threaded headsets over the past 20+ years and never came across one with a separate lock ring below the top nut so this design is a bit unusual.
I agree there appears to be a set screw in the notched ring that must be loosened before the ring can be unthreaded. The punch and mallet technique works well for removing notched lockrings if the right spanner isn't available.