All Observant hardware has its own unique serial number. In order to have this unit installed, Observant Support will need to know the serial number of your unit.
You can find the serial number either by looking at the unit itself, or if it's already been online in Observant Global, you can find it here
The serial number is next to its bar code on the lower edge of the C3, near the antenna connections and above the bar code (of the serial number).
An Observant Solo's serial number is also on the lower edge of the unit: