I found a great deal for a used Cintiq 21UX on eBay. Around $300 after shipping. Sweet!
It had one small flaw: The screen has a finger-sized dark patch/bruise toward the middle. It's nothing I can't work with; graphics show up in the spot; they're just dimmer. But when I saw an old NEC 21" monitor for sale on eBay for $30, I thought, "Hey, I wonder if I can swap out the screen?"
I did some research and it looked like the LCD panel might be compatible...
So I bought it! (I mean, hey, it was $30).
To cut to the end of the story: It doesn't work. It had jitter issues.
an NEC Display MultiSync LCD2180UX-BK LCD Monitor LCD 21" 1600x1200
It took about an hour to work out how to tear everything apart, (the Cintiq is a well-built machine! Very tight tolerances, and none of those annoying clips and plastic catches. If you unscrew everything, the back and sides pull right off, exposing the guts. Nice! I didn't have to break anything to get inside).
The NEC's chassis, screw placements and hardware sockets were indeed all compatible with the Cintiq 21 UX. The two screens swapped easily enough.
-And when I fired the replacement screen up in the Cintiq, it came on immediately, looking bright and beautiful. Ahh! But...
There was edge jitter. The Multisync's LCD panel driver board did not use the same chipset as the one in the Cintiq, and obviously has a different enough EM frequency profile that the Cintiq's shielding strategy made no difference and I got the standard jitter problem many DIY builds suffer from. -Stylus tracking in the middle was fine, but there was mad jitter along the top and sides. -Which further demonstrates that shielding is simply not the way to stop jitter; you *need* hardware with compatible EM frequencies that don't interfere with each other.
So I reversed the process, put the original Cintiq screen back in and put it all together again. And everything works now as before.
And I now have a spare 21" monitor. sigh.