the Swedish Volvo P1800: Designed by Pelle Petterson while working for Pietro Frua in Italy, built by Jensen Motors in West Bromwich England, and this unique one has an engine by Aston Martin chief Sir David Brown (thank you Doug!)

The cars were powered by the Volvo B18 1.8 liter engine with 90hp, that was according to Aston Martin chief Sir David Brown, was inadequate to power a proper sports car and he decided to commence a rather bold experiment. 

In March 1961 he commissioned his engineers to develop a 2.5 liter 4-cylinder engine shortened from an existing DB4 engine by cutting off two cylinders. The crankcase, crankshaft and camshaft had to be designed and built from scratch while the liners, pistons and valves could be taken straight from the six cylinder DB4 engine. 

After the first three engines had been completed, Sir David Brown made an agreement with the then UK Volvo importer Charles Singer to test one of the units in a Volvo P1800

The project was cancelled, the test mule scrapped and the remaining engines used for other projects.

Since 1974 these engines had been considered lost until, in 2003, engine #3 was found, overhauled, and an early Volvo P 1800 originally completed at Jensen's was found.

This result was completed in 2005

Since the two other engines were most probably scrapped many years ago, there remains only this one DP208 P1800, making it one of the rarest items in Aston Martin history.

https://www.classicdriver.com/en/car/volvo/p1800/1963/947778