I just happen to have an original print of this photo of the Italian trailer, it is from the IWM library.
From close examination with a magnifiying glass, the power unit is at the drawbar end of the trailer, it drives a considerably large compressor, probably a single cylinder and water cooled, it is as high as its radiator. My feeling is that it is for recuperator charging, high pressure air, for guns, tanks, etc., hence the cylinders stowed. Cannot work out the three rows of fittings on the side, unless, they are for charging smaller cylinders and those stowed are acting as reservoirs.
Richard
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