The answer is simple...

You will never be able to register your guitar and Line 6 is..."unable to "make-up" a serial number and add it to the database".

How do I know you are wondering??

I have had a JTV-59US prototype that has no serial number, but I do know it was the 40th of this model produced as it says so in hand writing inside the electronics chamber as well as inside the battery compartment, and even the hardshell case that came with it had green masking tape with the hand written #40 on it.

I asked this question all the time ago and the answer I wrote is word for word the answer I got back.  I never thought any purchased US made guitars would have no serial number as this seems like a bad situation on multiple levels.  Mine is a prototype so I understand why it has no serial number ID on the back of the headstock.  But you bought yours and it also has no serial number - that is strange in my opinion.

Oddest thing of all is that this super high tech cutting edge music tech company cannot add any serial number to their product database?  It seems that all units with serial numbers were issued before the products are produced.  Okay, but not being able to add any type of ID number?

I'm stumped, but after two years, I've just accepted it and click the "I'll do it later" option when monkey asks me to register my product.

Perhaps you can get a different answer then I got? 

Best of luck and take care,

Neal

PS - I just thought of this...Could you have been sold a prototype model instead of an ordered from Line 6 wait however long, US version?  I find it hard to believe Line 6 would make all US guitars without any ID number on the guitar in case it goes missing.  I think it would be worth a phone call from you to find out the correct answer, but it's just a suggestion.  If you were truly sold a prototype, do you think you're the only one to have ever had it in their possession and played it?  For the price tag of a US version, I certainly would wish to know.

i guess it depends on whether the OP bought a new or a second hand guitar.  if it was new then it should have a serial number.

Merlin's comment is correct, but as 99.9% of users on this forum are talking about production model guitars, and the number of beta guitars would be very small compared to the number of production models, it's unlikely you got a beta guitar... and if it's a new guitar it would be impossible that it was a beta guitar.

1st port of call would be talking to the store about this.  they may be easily able to trace back the guitar tot he supplier and have the S/N sticker sent out to you if it needs one