Not uncommon. Collosus was classified for 50 years. All examples were broken up and destroyed along with all the plans by order of Winston Churchill. After it was declassified years later, the few people that knew about it set about rebuilding it from whatever fragments of information they could scrape together through old notes that had survived and interviews with the original team. Flowers (the designer) was still alive at the start of the project, but I don't think he lived to see it working.
Apparently, when Collosus became public there were talks given where the Americans who worked on ENIAC (I think) were stunned to find that the British had solved a lot of the same problems they faced just before them. It was an extremely closely guarded secret.
The British and GCHQ kept the stuff secret for ~30 years. They kept it secret from the public only, the Americans knew, and the place was riddled with Soviet spies.
GCHQ also had invented the RSA algorithm before Rivest et al but kept it quiet.
