Following excerpt (and photo above) are from the book Hidden History of Denver.
In March of 1921, the Auto Bandit Chaser made its debut to help fight crime. It was a formidable piece of equipment built on a Cadillac chassis and comprised bullet-proof armor plate sides, a bullet-proof windshield, powerful spot lights, a large bell and a mounted machine gun on the front passenger side. The car was capable of holding six policemen specially trained in the art of riot control. Captain George Merritt aims the machine gun while the men seated behind him hold trench guns and high powered rifles with bayonets. Other men stand around the car. From left to right: Manager of Safety and Excise, Frank M. Downer; Chief of Police, Herbert R. Williams and acting Deputy Chief, Robert Carter. It is reported that when a policeman fired the gun during a demonstration, the machine gun rocked loose from its base and was ineffective.