The National Museum of Computing has finished restoring a Tunny machine - a key part of Allied code-cracking during World War II.
Tunny machines helped to unscramble Allied interceptions of the encrypted orders Hitler sent to his generals.
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.
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 machines were built following the work in 1942 of mathematician Bill Tutte. Plans were drawn up 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.
Prior to the creation of machines to do the code-breaking, the orders were broken by hand in what was known as "The Testery".
Bill Tutte's analysis enabled the development of the Tunny machine which effectively reverse-engineered the workings of the SZ42 - even though he had never seen it.
The first machine built to capitalise on Tutte's analysis was called Heath Robinson and the more reliable and faster Colossus machines followed soon after.
Tunny worked alongside the Colossus computer, which together with input from the Testery, calculated the settings of an SZ42 used to encipher 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 helped to ensure the success of D-Day.
"We have a great deal of admiration for Bill Tutte 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.