To start off, serial numbers can be used to identify two different model tools. Serial numbers can have groups, such as all serial numbers that start with "A" and all serial numbers that start with "B", would be a way of distinguishing between two tools that are identical, but one with cheaper parts. You are not going to find the exact model # and serial (lets call it group codes) on two different tools. If the numbers are exact, the tools are exact. First reason would be that it would be fraud to do so, and the second and more obvious would be, that there would be no way to identify the tool for future parts
From the beginning of this post I have been stating that the model numbers are not identical. HD products for example will use the same model number and add a letter or two, lets say Milwaukee 18V cordless drill Model #1032, and for home depot products it might be, Milwaukee 18V cordless drill Model #1032HD. If you had the two tools in you hand, they would look identical, but have different parts, such as maybe metal gears opposed to plastic.
If you're going to do the test not knowing this, you may just wind up comparing the same two tools. The best way to know that you are getting one of the good quality tools and one of the poorer quality ones, would be to buy one from Grainger and one from HD. We all know that Grainger sells the very best and charges for it.