Serial Number

Hi

I need to sort a warranty repair on my HDJ-X10, but when filling out the form, it asks for a serial number.

I cannot find the number anywhere on the cans. I've checked inside the ear pads as they say (but they have already been replaced) but there is no number on the pad. 

I've removed both ear pads, but there is no serial number anywhere.

What am I supposed to do? I can't even fill out the form to book a repair.