The Serial Number is on the barcode sticker on the cardboard box, the Certificate Of Authenticity that came with the headphones, and on the headphones in one of two places (depending on when they were made): inside the left adjustment block (near your ear when you're wearing them), or on a small sticker underneath the headband strap, off to one side and just above the adjustment block.
The serial number for Mobius can be found on a sticker on the box and also underneath the right ear pad, which can be removed by squeezing the side tabs together and pulling the entire ear pad assembly off. (To replace the ear pad, just line it up and press down until it clicks into place.)
The serial number is on the barcode sticker on the outer white box, the COA card inside or on the travel case, and also on the retail packaging.
The SINE Serial Number is on the barcode sticker on the box, the Certificate Of Authenticity that came with the headphones, and on the underside of the left yoke (the semi-circle that attaches the ear cup to the headband). Just rotate the ear cup to get it out of the way, and you'll see a small white rectangular sticker.
The EL-8 Serial Number is on the barcode sticker on the box, the Certificate Of Authenticity that came with the headphones, and is also located on the underside of the headband. Just pull down the headband strap and you'll see a small white sticker off to the side.
