Maybe it is a serial number of sorts, likely not that many made but im thinking for export purposes and possibly for the usa.
Although a harmless low powered toy in many eyes, but some countries have seemingly strange laws surrounding these things.
We know for the most part that webley tempests and hurricanes are numberless but there are a few with factory stamped numbers kicking around, I even had or may still have one myself.
There is one from another recent thread here, http://i739.photobucket.com/albums/x...e/DSC_8289.jpg
If you cant make a date from the stamping it may just be an I.D. to comply with certain laws.
Eric