But the advent of monocoque construction made it much harder to construct a unique car, which is why it's now such a costly business. But every so often a car maker will unveil a road-going machine that's unique, either as a customer commission, a marketing exercise or it could simply be as the basis for a production model which never sees the light of day. Sometimes it's a shame that only the one was made – but sometimes it's not such a bad thing: