chip that was used as a test sample . There's a number of different types, but the most common is the type that you show -- where the chip is hot-stamped like a normal casino chip and then over-stamped with a design (similar to the one you show). These test pieces or samples end up on the chip manufacturer's floor, hence the term "floor sweeping".
If my explanation was / is incorrect, I'm sure that someone will come along to correct me!! 
Steve Bedo
