Here is where you can find the serial number on the device:
Arctis: Inside Headband Near Earcup
Siberia 800: Inside Headband Near Earcup
Siberia 840: Bottom of Wireless Transmitter
Siberia 650: Inside LEFT OUTER Ear Plate
Other Headsets: Inside Headband Near Earcup
Keyboards: Bottom of Device
Mouses: Bottom of Device
Controllers: Bottom of Device OR Inside Battery Cover