Back when the 737 was introduced there were no electronic primary or standby instruments. The standby instruments in those days were typically a mechanical attitude indicator and a separate IAS/Altitude indicator. The same was true when the MD-11 was introduced. At that time Honeywell and DAC believed there was so much system redundancy that no standby instruments were required. In all the Honewell advanced flight decks a PFD and Engine display remain after failure of any 4 of the 6 display units. Standby instruments were fitted because that was easier than arguing with FAA that they were not required.
Modern electronic standby instruments have much more capability but there is a difference between capability and a requirement to have that capability.
Last edited by EXDAC; 16th June 2021 at 12:25.