BPB644 is the original Dennis factory Big 4 demonstrator. It was built in 1931 with the D3 side valve engine rather than the later OHV engine. When it was with the factory it was painted green with a blue bonnet and 'extra chrome' was specified on the Works Production Order. The locker handles have 'Dennis' cast into them and it is believed to be the only known occurrence of such items. The photograph shows the Prince of Wales having a test drive in 1934. In 1936 this machine was sold to Long Eaton Fire Brigade in Derbyshire, where it saw considerable war time action (See below!).
It was sold out of service in late 1950's and remained with a private collector until the present owner purchased it in 1995. It as at present undergoing a full restoration.