Wasn't on one in Vietnam, but have been on modern ones. Basically has a standard M577 interior. For strechers, there is a hanger system that goes from the roof to the floor. It has hard rails on the outside sections with hangers at the front and rear, in the center of the vehicle, it has chains that hang from the roof racks to the floor with hangers attached to the chains. There are 3 strechers on each side, total of 6 carried.
Here is the hanger system on a M113 Ambulance, it is same in M577, just longer rails and chains.
I have built an M113 version, pretty easy and came out pretty nice, no pics though.
Hope that makes sense to you.
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