Unity 450F

I need support on this appliance urgently.  Support keeps asking for the service tag which I have in an email as a sequence that starts with APM.  But I believe this to be the serial number, even though I have emails stating that this is the service tag.  I have pulled out the appliance as far as I can without turning off the appliance and am unable to see the service tag anywhere underneath or on the side of the appliance.  Could someone please tell me where this service tag is so I can get assistance?  PLEASE!!!!

Steve