I honestly don't know. I am a software/networks guy, I just forwarded your info to a hardware guy who happened to be here in the test lab, what I posted was word for word his response.
ROS Window. Laser is not writing on the drum in that location. If drum and developer have been ruled out and it's just one color.
We have a 7435 we purchased used as a backup to our main copy station. Recently developed a white line (void of Toner) on the magenta only, line is about 3/16th in wide. Swapped in a new drum with no effect, cleaned print heads, no effect, ibt belt is fine. So we swapped in a brand new developer housing (you could see a void on the mag roller with a flashlight). Once it was primed up with toner it developed a line in the exact same location. With a flashlight shining in through the drum opening you can clearly see a void on the mag roller just like the initial developer housing it replaced. I'm stumped, what should I look at next, and what could cause two different developer housings to develop a void (very quickly) in the same location on both mag rollers.