Another thing that has not been mentioned is that the IBM AS400/iSeries computers derive from computers that had reel to reel tapes (for a visual example see the computers on the oil rig in "Diamonds For Ever"). This type of computer is completely different to the PDP11 or Vax, and is very much still in use in the U.K. financial services sector.
If you are based in the U.K. a good resource is the National Computing Museum at Bletchley Park.