bubbers 44:
A long time ago a 727 was lost at night taking off from LAX at night with two generators and another was lost overloading the final one and losing everything. That might have been the reason for standby instruments as we have now.
I thought all 727s had standby power in case all three generators and essential power were lost. Standby power had to be switched on. It powered the captain's normal flight instruments and I believe one VHF comm and one nav comm from the battery bus. UAL's airplane that went into the ocean departing LAX had standby power but the stock Boeing position for the standby power switch was on the F/E panel. TWA paid to have it placed on the overhead panel where all three crew members could get to it.