Well... It looks like I was wrong (serves me right for posting before I've had any coffee <g>).
The supply voltage does not work at 5v like I suspected. It looks like it is designed for 24v but will run with an i-Opener power supply at 19v (everyone has one of those lying around <g>).
I am probably also wrong about the RS-422/485 stuff. I've found sites that sell adapter cables to adapt the ICL-9520 to RS-232. Examples are here and here.
It looks like this display was used as part of a POS terminal scanner/scale system. The VFD modules are likely a custom pieces designed for ICL. This would explain the lack of data sheets and limited interface.
There are actually two part numbers and the front and back displays are somewhat different. Only the front has an external connector. There is a second connector that links the two boards. It still may be possible to use the displays independently, but it will take a bit more hacking.
I'm hoping that the control commands are common with the Futaba displays for which I have info. I have a few machines that have a RS-422/485 ports and a RS-232 to RS-485 converter. The next step is to try to send data to the display.
Electronics run on smoke, if you let the smoke out they won't work