https://antiqueradios.com/forums/viewtopic.php?t=157835

During my many years of owning a large factory service center, no tool was quite as helpful as factory service bulletins. I submitted a couple, but having them saved time and a lot of headaches.

Hmm, that post on ARF is from 2011. I think I probably read it then; I know a contrast/brightness modification for the CTC-5 was a well-known improvement "back in the early days". That must be the one. Thank you for posting it here.

In a similar way starting with CTC12 chassis, RCA tied the CRT cathode bias to the shunt regulator grid.
This allowed shunt regulator to be cut-off during extremely bright scenes, regulate only during darker material to maintain load on HV supply.
It seemed to reduce a "blooming" issue with a CTC11, was tough to mitigate with just HV, brightness/contrast controls.