Richie,
My Whisper has no other numbers apearing anywhere other then those on the left breech - and the factory rated pellet speeds (ha,ha!) and 5.5 mm (.22cal) on top of the reciever.
I'm assuming those numbers in sequence on the left side of the breech are Gamo's model, serial and date of manufacture code all-in-one. Since modern rifle model and serial numbers are suposed to be in plain sight so they can be easily recorded without need of any disassembly (which may also affect warrentee), where else would they be

?
Another reason I say that is because my Whisper's number code ends in -08 and I just bought it end of Feb 09.
Paul.