serial number

Cannot find it on the housing, in the battery compartment, where the foot mounts or under the connector cap... or in the manual... What am I missing .... where is that serial number located?

I recently bought a second stage octopus from Hollis. I was planning on registering it but the form says the first stage serial number blank is mandatory and the second stage serial number blank is optional. Do I just put the octopus serial number in the first stage serial user input area...