Serial number will be on the Base engraved into the POM Plate between the Battery Cap and the Bottle Cap.
"Limelight" will be engraved on the Body (If looking from above the switch Plate it will be on the Left side at a slightly lower level than the Squonk Bottle opening.
Freehand S will be on the Switch Plate.
Depending on the age of the Mod, if it is a Mosfet Model, Piezo will be engraved on the 510.