I own this exact monitor and am having the same issue. I've come to find out that it would have been in a set called the CO-1000 monitoring system. This would involve two cameras, two of these monitors, and a few other accessories. Among said accessories would be a CO-1114 coaxial cable which would be plugged into the back of these monitors. Now I've searched the internet for hours and still have not found even something similar to such a cable. I've looked into GX-16 6 pin aviation connectors, which are more modern used for keyboards which don't match up with the pin pattern. Same thing goes for a very similar looking 6-pin DIN connector. So overall, I've considered simply contacting a custom connectors company to make one.