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As per T/O & Es Medical Detachments had NO Ambulances . Furthermore, again as per official T/O & Es, Glider Infantry Regiments + Parachute Infantry Regiments + Airborne Medical Companies, had NO Ambulances either !
The ONLY units that were provided with Ambulances (Dodge 3/4-Ton Trucks) were the Collecting Companies pertaining to the Medical Battalion (however the Abn Div only had a single Abn Med Co) .
When you mention the Ambulance illustrated in Mike DT's Book re Doc McIlvoy, this MUST be an exception ! Not everyone followed the AR and T/O & Es, and I guess Airborne personnel must have been anxious to try and get hold of extra vehicles, one way or another ... you should not consider this to be the norm (markings too are not consistent with AR)