In the years 1958/59 and 1961 four functional models and two production prototypes were built.

The production prototype flew on 6 September 1940.

A final production prototype was shown at 1967's Turin Show.

K-2 was the production prototype for future K-class airship purchases.

We'll have production prototypes in about a week.

However, in March 2013, production was secluded for 2016, with a production prototype due by 2014.

Early testing began in 1954 and production prototypes were available the next year.

It was first flown in April 1939 and led to improvements on the second production prototype.

Energy Partners plans to build a "proof of concept" car, rather than a production prototype, running on two hydrogen fuel cells.

However very few small capacity engines did eventually implement such a scheme in their production prototypes.