Quoted from RCA1:Put the translite in the bracket below the monitor. It's not as clear as the regular playfield glass, or a proper service mirror, but you can see the monitor fairly easily reflected in the translite glass.
This! The game was actually designed to do just that. It is why the backglass fits just right in that space above those pegs. The whole idea was to use it to reflect the monitor while servicing the game. That was such a great design idea. Really the steps they took to make the pins easier to service were really impressive at the time and one of the best things about the P2K platform.