Nikon has long used the first digit of the serial number to indicate region for many of their products:
- Not used
- Europe (excluding UK)
- Australia/New Zealand
- Asia (excluding Japan)
- United Kingdom
- Not used
Initial production of a product is almost always a two-digit variant of that with the second digit being zero (e.g. 30xxxxx in the US). In a few rare cases, Nikon has used digits outside a region when production exceeds 999,999 units. That's why the second digit isn't just part of the serial number itself: its reserved as a special region indicator, when needed. In other words, while US products normally would be 30, 31, 32…39 in initial numbers, Nikon has set things up so that they could make, say, a 59 region code that's US for a product (normally that would be Canada).
Note: with the D7100, Nikon started using the 25xxxxx code for US bodies; it appears that they're adjusting to give them plenty of room for US bodies, and since the US number came after the much smaller Japanese market number, it makes some sense to "steal" some of the Japan numbers. But also oddly, the other country serial numbers don't tend to have a 0 as a second digit, which is really a strange departure for Nikon. We'll need to see some new camera models appear to see if this is the start of a new numbering scheme for Nikon.
A few products, most notably the top end pro cameras, use only the Japan serial numbers. Thus, you have a D3 with a 20xxxxx serial number no matter where in the world you bought it.
The same scheme usually applies to lenses, though many US lenses are prefaced with the letters US these days.