The engine block gives the game away.
Early in the development of the McLaren F1 - Mclaren needed some vehicles to act as test mules for various components. They purchased a number of Ultima rolling-chassis - and then proceeded to fit them with test engines and other running gear.
They were supposedly scarily fast - and very VERY loud - hence the lash-up muffler fitted to the mules.
I believe they were dismantled at the end of the programme?
Imagine if you found one of those with the Macca F1 engine still fitted!