A year and a half ago, I wrote up a post lamenting the problems caused by counterfeit USB to serial adapter chips.
One of the primary issues was the inability to tell the adapters apart on the USB device tree when hooked up to the same computer. (There were, of course, other problems.)
Amazingly, after revisiting the thought recently, I discovered that any of the cheap adapters built atop the Silicon Labs CP210x chipset would in fact allow you to set the serial number to a value of your choosing!