This might be hard to date.
1. It's not the earliest Korean Made ones - they had TRIBUTE on the bridge and in big letters on the headstock - unless the bridge was replaced with a US one or later Tribute one, and the decal was removed.
2. It doesn't match the Indonesian made ones - I believe all current Tribute models have the "G&L" in white letters, not gold,and they say "Tribute Series" not "Guitars By Leo"
3. The "SB-2" looks just slightly different to my eye from the current Indonesian model.
3. It doesn't show the serial in a circular pattern closest to the tuner adjacent to the nut on the back of the headstock like the Indonesian ones.
4. Looking here:
http://global.rakuten.com/en/category/c ... c_bass/gl/ and here:
http://www.kurosawagakki.com/sh_bass/gl ... D%8C2000je shows some Japanese Tribute models with the Gold "G&L" but I haven't found a closeup photo that shows whether it says "Tribute" or "Guitars by Leo" on those.
So, you might really have a Japanese Tribute.
In any case it looks like someone either
a. removed the serial
b. Japanese models might not come with serial on the back of the headstock?
c. Removed all the decals to a Japan or Indonesian Tribute and added homemade decals that look quite good.
d. Added a US neck but removed that serial too.
Good luck with the mystery! You can also try bassesbyleo.com and see if anyone there knows.