The instrument's serial number for the i1Display Pro and the ColorMunki Display is located on the sticker on the permanently attached counter weight.  The ColorMunki Display's serial number begins with "CM..." whereas the i1Display Pro begins with "i1...".  Additionally, the serial numbers can be accessed by plugging in the devices and then do the following:

ColorMunki Display:

Open ColorMunki Display with the instrument plugged into the USB port.  Select "Help" in the upper left and then "Register".  Click on "Phone Registration" to display the instrument's serial number.

i1Display Pro:

Open i1Profiler with the instrument plugged into the USB port.  Open "Licensing" in the lower portion of the home screen and then select "Registration".  Click on "Phone Registration" to display the instrument's serial number.