It is the Testors Hot Metal tandem axle trailer. It is 98% metal, a few small parts are plastic. Also, it is 1/24 scale, so a bit over sized for my favored 1/25 cars.
That is why I have the sheet of styrene across the floor of the trailer. To me, this makes it appear a bit more realistic with a 1/25 car. I still need to paint the styrene sheet and probably the fenders, too.
It has been discontinued and I don't see any available for a reasonable price when conducting a brief search.