The first production episode filmed was "The Corbomite Maneuver" and in it was shown a display on the bridge reflecting the coolant temperature. Interestingly enough that display was not actually one of the display on the bridge at all; it was a one-off created only for the close-up and it was never seen again. Here is a recreation of that display:
Of note is the red "X-1000" and the "X-100" on the right side of the display. These are not really seen in the shot but if you lighten the screen cap they appear!!! Below is the screen-cap and a copy of it with the brightness cranked!
The screen as seen in the episode:
The screen with the brightness cranked up, notice the lettering in the circles:
A close-up of the stealthy lettering: