in the pics, i see the adapter for that vga cable connected on both ends.

that "loose white end" is an option for a different monitor, i've seen that many times with VGA/LCD upgrades or hacks.
or it's the original RGB input, not compatible with the monitor in that machine now.
why the vga cable is even in use for this setup, i have no clue. there are much better adapter choices or modification
choices out there, and honestly, not needed if you use the correct tube for a 1992 jaleco cabinet.