OK, ... I'll bite. What are they for ???
I'm guessing - could they be rigs used to oil roads? They each have a tank that could be used for road oil and the enclosure on the back could be a shield to control overspray.
They are portable showers for little people.
Could it be for asphalt road repair? The unit on the back would be for heating up the existing asphalt road.
Not likely for asphalt repair or maintenance. Yes there were asphalt roads at that time. But not very many of them as miles go. Most of them at that time were still only a couple years old, although poorly made.
I suspect those rigs are for oiling the road as Henry P says. To control dust, spraying the roads with a tar-like oil was common in many parts of the country. I remember such roads around Henry's area when I was growing up. My grandfather raised peaches outside Modesto throughout the '30s through the '50s. I even remember seeing similar oiling rigs around there (complete with the "skirt"). I think they pretty much stopped oiling roads in about the '50s. I remember asking my grandfather about some of this when I was little. He told me then that they "aren't doing that anymore". There were several of those roads in his area. I thought they looked neat!
Most "paved" roads prior to the mid '30s were either brick, or cement.
Drive carefully, and enjoy, W2