Tunny machines helped to unscramble Allied interceptions of the encrypted orders Hitler sent to his generals.
The working replica of the Tunny machine in actionThe National Museum of Computing has finished restoring a Tunny machine - a key part of Allied code-cracking during World War II.
The rebuild was completed even though almost no circuit diagrams or parts of the original machines survived.
Intelligence gathered via code-cracking at Bletchley underpinned the success of Allied operations to end WWII.
Time synch Restoration work on Tunny at the museum in Bletchley was re-started in 2005 by a team led by computer conservationists John Pether and John Whetter.
Mr Pether said the lack of source material made the rebuild challenging.
"As far as I know there were no original circuit diagrams left," he said. "All we had was a few circuit elements drawn up from memory by engineers who worked on the original."
The trickiest part of the rebuild, he said, was getting the six timing circuits of the machine working in unison.
The Tunny machines, like the Colossus computers they worked alongside, were dismantled and recycled for spare parts after World War II.
The first Tunny machine was built in 1942 by mathematician Bill Tutte. He drew up plans for it after analysing intercepted encrypted radio signals Hitler was sending to the Nazi high command.
These orders were encrypted before being transmitted by a machine known as a Lorenz SZ42 enciphering machine.
Bill Tutte's work effectively reverse-engineered the workings of the SZ42 - even though he had never seen it.
Tunny worked alongside the early Colossus computer, which calculated the settings of an SZ42 used to scramble a particular message. These settings were reproduced on Tunny, the enciphered message was fed in, and the decrypted text was printed out.
By the end of WWII there were 12-15 Tunny machines in use and the information they revealed about Nazi battle plans aided the Russians during the battle of Kursk and helped to ensure the success of D-Day.
"We have a great deal of admiration for Bill Tuttle and those original engineers," said John Whetter.
"There were no standard drawings they could put together," he said. "It was all original thought and it was incredible what they achieved."
One reason the restoration project has succeeded, said Mr Whetter, was that the machines were built by the Post Office's research lab at Dollis Hill.
All the parts were typically used to build telephone exchanges, he said.
"Those parts were in use from the 1920s to the 1980s when they were replaced by computer-controlled exchanges," he said.
Former BT engineers and workers involved with The National Museum of Computing have managed to secure lots of these spare parts to help with restoration projects, said Mr Whetter.
The next restoration project being contemplated is that of the Heath Robinson machines, which were used to find SZ42 settings before the creation of Colossus.
That, said Mr Whetter, might be even more of a challenge.
"We have even less information about that than we had on Tunny," he said.
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