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[quote=Jim Oxley;93249]
Any idea why the card index would have been destroyed? Surely not for security reasons.[/quote]
There were rumours that the card index was kept in a basement at the Admiralty (and other locations were also suggested). If so, it is probable that it was destroyed at a later date. As far as I can find out, there ts no reliable 'paper trail'.
Churchill was quite clear in his instructions to protect the existance of ULTRA at all costs which ultimately is at the heart of our research problems today. It also gave problems to the Intelligence establishment in the 1950s as the best of the analytical equipment (including Colossus) had been destroyed in the same spirit. The details and references contained in the card index was a 'smoking gun' piece of evidence that the Allies were able to read machine encrypted radio/teletype messages and that was as big an issue in the Cold War as it had been in the Second World War. We now know that the secret was out, but that is hindsight.
Hope this helps,
Bruce