[Penthode]

It does not appear to be a horizontal drive line. The line is most likely due to insufficient horizontal retrace blanking. During horizontal retrace there may be a positive overshoot of the rear of the sync pulse.

If it was a drive line, horizontal scan linearity would be affected with evidence of picture foldover across the line. I see no evidence of this in your photos.

To confirm, adjust the horizontal sync phase by adjusting the horizontal hold control. If the line moves, it is due to a blanking problem. If it stays fixed, it is a drive line.

You should be able to minimize it by adjusting the fine tuning. But there are means to improve retrace blanking if that is the problem which I believe it is.