I have a Slash amp that was missing the serial number sticker on the outside.
I pulled the Chassis out to have a look and there was one in there. I am pretty sure they put two serials on the amp. One on the outside and one on the inside.
Picasa Web Albums - Richard G - Slash Amp Gut...
I blacked out the serial number on the image
I wonder If Marshall forgot to put it on your reissue???
With the Cab the serial sticker in usually on the plastic plate where the inputs are.