From searching the forum here, it appears people have encountered it on the 170, 180, Duo 280, 540, etc... It appears all the early monochrome active matrix monitors were impacted by it, but whether or when it happens to a given system seems to differ significantly.
All of them had the same symptoms, darkening at the four corners that tended to get worse the longer the affected system was used, and it persisted even after the display was turned off, but would eventually disappear if left alone long enough, with eventual deterioration to the point where it swallowed the entire screen. It seems to be a progressive and terminal condition once it appears.
I know in one of the threads someone has good pictures of what it looks like.
One can easily discount the backlight since that would have nothing to do with darkening after the system was turned off, and caps don't seem to be an issue, since that was ruled out in yet another thread on the topic. It seems to be some kind of defect in screen itself somehow.