From MetroAmp Wiki
The red rocker switches found on '75 and up Marshalls changed from 5 terminals to 4 terminals. At which time the connections for the internal neon lamp were tied to the contacts of the DP/ST switch.
Here is the original 5 terminal wiring:
To replace the 5 terminal switch with a modern 4 terminal type, simply omit the extra 120V AC source wire (typically black and wired to the end of the PC board).
Marshall often wired the extra terminal to the 110V AC primary tap of the PT. This provided slightly less voltage to the lamp and I assume it was done to make the lamp last longer.