How to uniquely identify USB HID class device?
I think it will vary by manufacturer.  There is a serial number field, but many don't fill it in.  USBDeview is a free app that will let you peek at all the fields for all the installed devices (http://www.nirsoft.net/utils/usb_devices_view.html).  A screen capture for an HID device on my system is attached below. Install it (actually, no need to even unzip, it runs as an executable), and see if there's a unique ID in the devices you're talking about.