Just to wrap things up (pun intended).  When pilferage became a common problem, only loads in large containers (55 gal drums and large wood crates) were shipped this way and solid-top trailers became the norm for smaller articles.  Tarped trailers are still used for some bulk commodities, notably grain, or on rolloffs and dump trailers carrying dirt and rock (although not trailers - dump trucks as well).  If a dump truck has a tarp on it, it sort of dates the layout to the 1970s and later.  A 1955 dump truck would not likely be covered.