I agree that's a siren on the front. As I remember, most ambulances were converted/dual-purpose hearses (had a way to lay someone on a gurney and could go fast what with those big cadillac and pontiac engines - I was an EMT on ambulances in the early 70s) up until the late 60s/early 70s. My guess (and that's all it is) is that it's some sort of special purpose fire truck or a police vehicle; maybe a paddy wagon. Anyone?
