What you have there is an almost complete bench test rig for any generic JAMMA game board.
The concept behind this is if you had a machine that didn't work, you would pull the board and plug it into a rig like that to see if it worked outside of the cab, thereby determining if the board was at fault, or something else in the cab. You would also use it to test boards after repair, or plug in a known good board and use it to drive a monitor you were working on.
It looks to have a +/- 5 and 12 volt supply, board connectors and ISO trans for the monitor.
You appear to be missing basic controls (buttons to test the board I/O), a method to "coin up" the board (more buttons usually), and speaker(s)
In addition, you need some sort of monitor to complete the setup.
I'd guess those molex connectors on the bracket was where the buttons and such plugged in.
Check that power supply carefully before attempting to plug a board into that, it's an old analog supply, they can be flakey when they get old.
You got some decent parts there, but you need a few things to complete the rig.
These rigs were usually built by the somebody that needed one, often out of old game parts (that's what this looks like). So I can't give you any specific details on what should be connected to where.
If you obtain a basic JAMMA connector pinout diagram (quick google should get you one, or at Bob Roberts site), you ought to be able to figure out the connectors pretty quickly.
D