I don't have that on my Widelux or Kodak Panoram, but I do get such an artifact on my Cirkut - *if* I don't warm it up with 1-2 360-degree practice laps before actually shooting. Come to think of it, I normally warm-up my Widelux before shooting too, by rotating the lens manually from RH side to LH, release, RH side to LH, release... about a half-dozen times. I don't know what that would do (pro or con) for your Horizon, though.
Reason for the dark column: Something is momentarily slowing down the rotation when you see a dark vertical bar like that, but I don't know enough about Horizons to tell you *what* is slowing it down.
On my Cirkut, I'm guessing it's lubricant that's thickened through the years, and needs to be warmed up.