HP 39gs without serial number – worry?

[metawops]

Greetings fellow HP calculator nerds! ?

I just acquired a "new" (not really) HP 39gs which was missing in my little collection.

I noticed very soon that the serial number sticker on the back was missing. Strange ... but maybe it just got loose.

However, when I did the "hold + and - and the reset button for app. 3 seconds" trick I saw this on the screen:

   

So the calculator actually has no serial number! Bummer! How can this be? What did I buy? Is it genuine HP? How would I know? Do I need to worry? Or do I have a rare special collector's item? ?

Curious ... Any hints?