@Swerve is correct. They're Round-wounds that are then polished down until they've removed enough metal that they're almost flat to the touch. They definitely feel different from flats. They're not polished so far down that there's no divot where the windings meet. But they're no longer round windings. They play, essentially, like rounds. They just feel and sound slightly different from standard round-rounds.
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Edit to add: I've played "Half-rounds" many years ago in the past, maybe the mid/late 1990s?. Don't remember what company made them. I remember them being very abrasive on my fingers. These are similar in concept, but VERY SMOOTH to the touch.