Some notes I had while reverse engineering the CRT in this image. They reflect how I came to the conclusion in ./pinout.txt. Note that it flips the image horizontally, as it's intended to have a mirror as the surface your eye faces in the viewfinder unit.

What are the chances this thing isn't composite but Luma / Chroma? 
Even though it's not colour.
The camera it came out of is dead, so I can't check there.
There are two lines coming in for video, I think.
All the controls for brightness and such are on the board there, so nothing should be going back towards the camera.

It does power up, but it's fully bright.
So, I tried to give pin 1 a luminance signal, and the tube... turned off?
Huh.
Giving Pin 3 luminance does... nothing.
I probed for voltage, and Pin 1 has 8 VDC on it...
VCC is 11.7 VDC.
Could the tube control board be expecting the input signal at 8 VDC?

Turning the brightness down on the board lowered that voltage.
Now I'm real confused. 

Got it.
Firstly, the Vertical and Horizontal sizes are completely fucked.
Secondly, it's Composite with the typical 75 Ohm impedance.
Thirdly, it requires the composite ground to be linked to DAG ground.
Since this is a DC CRT, that means VSS in this case.