It's a 1939 Mercedes Benz W154, sometimes known as a W163 though it's actually a 1938 W154 with the M154 engine replaced by the 1939 M163 unit.
During the Second World War Mercedes Benz had stored its racing cars in and around the factory at Unterturkheim, but later in the war when the allied bombing became more of a threat many of the cars were taken to other, less targeted places for safety. They were usually stored in pairs and this car, chassis W154/10, together with W154/9, was walled up in the workshops of Czech pre-war racing driver Antonin Vitvar in Stará Paka. After the war W154/9 ended up in the USA and W154/10 was handed over to the National Technical Museum of Prague. Many years later the car was sent to Tom Wheatcroft's workshops at Donington Park for restoration and then had a period on loan at the Museum.