'Is not the sensor itself is a single element, one-dimensional device?' I assume so, that is why I asked for a take away part guide.
'What was the failure mode?' We tested many different light pen programs, including Atari Graphics cart and that SALT 2.07 cart. All have that problem.
'Did it appear to detect the exact same position all the time on the broken axis?' Yes!!! Totally forgot, thank you so much, it was frozen in the lower left corner! Good point!
Have made a photo of the light pen head, please see below.
Hmm...frozen in the lower left corner does not equate in my mind to working in one axis.
That sounds like working in no axes, possibly not detecting the light at all. Which would lend credence to the theory about the brightness of the screen. Or maybe the sensor became less sensitive due to optics or electronics. Maybe the sensor failed entirely.
If you knew more about the electronics, you could probably build a static test harness fairly easily. Knowing the pinout of the device would probably be enough to give that a try.