We had some at work like that for the CNC Mills to store their tooling. They were fairly large for the areas so they decided to get rid of them. The ones we had were bought from Grainger maybe. I can't remember right off, but one of my coworkers wanted a roller. We cut the welds out of the main frame and slipped the box out. Pulled the casters off of the main frame and attached them to the bottom of the roller. I want to say that the box was along the lines of a Waterloo, or a Husky.
The roller the OP is showing is the mating of a cheaper box and an angle iron framework, but they are bought that way. They are not high end boxes.
Question for the OP.....does the box have ball bearing slides? Also I fairly certain the back is open to the inserted box. Look at the top of the inserted box and see if there is a sticker or a stamped name or number.
Can you turn it around and take a pic of the back of the box?