It looks like it's an endload hearse that someone tried to make into a combination. The giveaway is that there are no attendant seats, and the rollers don't look to be the reversible rollers. It also doesn't have the small medical cabinet in the divider, though that's not necessarily on 100% of the combinations. The stretcher looks '80s vintage, and the oxygen tank is just on a removable rolling cart.
The siren (Model V, I think, someone help me out here) is a Federal but it's the smallest of their sirens, something used by volunteer firemen on their cars. If it were a real combination, it probably would have had a bigger siren, and probably under the hood.