The GXW-56 is not a very limited model probably, hence the release of a Non-Atomic version in the US and probablt other parts of the world. Serial numbers or numbered models are pretty rare. I believe there are no G-Shock models with serial numbers anymore. The GW-200 versions were the last. You can find batch numbers on newer models though. You can see one on the back of Feisar's GXW-56KG.
Note also that retail prices in the US are very low compared to the retail prices in Japan. It might be that the GX-56KG will cost around $180 or less, with only the Atomic TIme feature missing.