Without seeing the Interior of the cars, its impossible to tell if it was a Combo or an Ambulance to start with..
The '59 M-M Low Top Limo Style coaches ALL looked exactly the same on the outside. Ambulance, Hearse, or Combo would all look identical from the exterior. The main differences were in the exterior Paint, and sometimes optionsl warning lights.
Judging from the photos that I did see of the interior of both cars, I would tend to lean towards the feeling that they both started out as Ambulances.. (The interior configuration and seat style that I can see in the Photos matches Factory Literature that I have for the 1959 Low Top Ambulance) Although with a better view of the floor and the interior I could be more certain. The Floor of the Ecto-1a seems to look more like an ambulance floor than a Combo floor from what I can see based on the compartments that are there.
TheEcto1a