ChristianJ I was perhaps recalling the Hermes 4a, where the emergency standby instruments were from a constantly recharged battery ( 24 volt ? As a pilot I was not entrusted to use a voltmeter !) On checking, my Type Rating has now expired.
Precise instruments used to be backed-up by simpler, more reliable, if not quite as precise, earlier generation ones - think of any remote indicating Compass, which would be backed up by something that Captain Cooke would have recognised, subject as that would still have been, to more magnetic deviations and acceleration errors.
I carried a load of Rhodesian Riflemen to Singapore. They had not flown before and asked to see the Flight Deck of the Hermes. The plywood door was enough security, then. They reported back that we :
" had a hundred and twenty twelve clocks, all telling different times..."
which was poetically accurate - but I never counted to see if they were right.