Here are few screencaps from different sources:
Bleach 297, where 16:9 episode actually used 4:3 material in one reminiscence scene (they even animated the black bars) - poor source material?
Xena the Warrior Princess:
Finally a Youtube video:
If you look closely, each of these frames contains a thin black line near the bottom, or in the case of the Youtube video, near the top. The line spans about 60% of the video width. What is it and why is it there? Is it used for some kind of calibration?