It all depends on the certification process for each aviation authority. It is quite common for one authority to insist a manafacturer provides an different fit for operators under thier system.
But essentially what has been said here is correct. However, there can differences within fleets. My company has a fleet of modern Fly By Wire aircraft which were ordered at different times. This type has three ADCs (air Data Computers) which compute airspeed, altitude and rate of climb and feed them to the IRS (Inertial Reference System a very accurate navigation system which used laser ring gyros to compute attitude and position information) which then feed this ADIRS information to the displays in front of each pilot. Normally, the left seat occupant gets the info from ADIRS 1 and the the right seat from ADIRS 2. If an ADIRS fails, the info from 3 can be switched to that side. Also, there is a standby altimeter which in the older aircraft is a standard pressure instrument, the newer ones another minature LCD dispaly with a composite attitude/altitude/speed display - much like the main displays. But the really old aircraft also have a 5th altimeter in meters, as we do a lot of flying in China which uses the metric system and these aircraft don't have meters information on the main displays.