This is an early concept model used during the design of Spottswoode's Lamborghini Diablo stretch limousine. This particular model is made from an existing Lamborghini Countach model kit with the 'stretch' part custom made by the model making crew. Somewhere during the production they switched from a Countach to a Diablo.

Here's a shot of the back clearly showing it's a Countach

The final design was a Diablo however: