Although not exactly a bus, and not the main issue by it's small size, its close. Information on the 1 ton chassis is a rare find. This picture can be found on the website of fire department of Bremerhafen
An extraordinary well documented story with a lot of details on firefighting and ambulance services. An extract:
From December 1933 all the fire departments were incorporated in the police. The phrase "Feuerlöschpolizei" (fire fighting police) emerged. A busy service: in 1935 1807 transports where undertaken with two cars, by the Wesermünde professional squad. In 1937 a new ambulance was bought, a 1 ton Opel-Blitz for the price of 8.500,00 RM (a small fortune). It's color is dark green, and it has its service written on the doors: "Städt. Feuerlöschpolizei Wesermünde" with a licenseplate "pol-5751". Pol is short for "Polizei" (Police).
Services seized by lack of staff and are taken over in March of 1944 by the German Red Cross.
The occupational forces ordered the fire department to start with the one remaining ambulance the service in 1945. Maybe it's the Blitz in the picture