Historical question: Longevity of Colossus.

[Paul Leyland]

I visited Bletchley Park on Friday as part of the Microsoft Research works
outing.  Tony Sale showed us around and mentioned something that I'd heard
before but never thought about.  For those who don't yet know, Tony was the
chap who rebuilt a Colossus at BP.

Tony said that all but two of the wartime Colossi were demolished, the
remaining pair being carted off to Cheltenham where they remained until they
were destroyed in 1960 or so.

My question: why did GCHQ keep *two* of them for so long?  If they only had
one, it could have been an exhibit in a museum.   Keeping two bits of kit,
each occupying several cubic meters and requiring 5.5kW of power to run
suggests that they were still useful.

Does the answer to this question have any relation to the observation that
the design of Colossus was classified until very recently?


Paul