Early code breaking effort

BANBURISMUS: Banburismus refers to an optical aid to comparing two messages against each other, looking for characters which match in both. It is done by placing the messages (represented by holes punched in sheets of card) on top of each other over a light-box and counting places where the light shines through. Light can only shine through where two holes are in the same place - representing matching characters at that position in the messages. ( It seems that there was a variation on this theme where the two messages would be put on top of each other over a dark table-top. Where the holes lined up the dark table-top would be visible and would contrast with the light-coloured card. ) The card sheets were printed in Banbury (a town in central England, not far from Bletchley Park) and were known as "banburies" - and the technique was "banburismus".
You might Google this for further information on the early code breaking efforts.