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 was almost always a two-digit variant of that with the second digit being zero (e.g. 30xxxxx in the US). In a few cases, Nikon has used digits outside a region when production exceeds 999,999 units (see Note in next paragraph, though). 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: it appears that Nikon hasn’t changed the number of digits in most serial numbers, which as product quantities increased in the digital age, has caused Nikon to have to abandon the clear logic they formerly used. For example, with the D7100, Nikon started using the 25xxxxx code for US bodies; it appears that Nikon has been adjusting many of their previous serial numbers to give them plenty of room for US and European bodies, and since the US D7100 number comes after the much smaller Japanese market number, it makes some sense to "steal" some of the Japan numbers. But also oddly, we’re now seeing that a lot of other country serial numbers don't tend to have a 0 as a second digit, which is another departure for Nikon. As I update this article, Nikon has slowly shifted to a very problematic (for customers) method of uniquely identifying region, especially in Europe, where they’re using previously Canadian and Australian numbers (e.g. 51xxxxx and 61xxxxx).
A few products, most notably the top end pro cameras, use only the Japan-based 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.
So where we stand is this: older gear clearly follows the above practice. The latest gear is much more complicated in how serial numbers are used. That said, a serial number with 0 as the second digit is almost always allocated according to the table, above.
As I noted two decades ago, Nikon really just needs to abandon this serialization practice. The primary reason it’s done is to enable Nikon to identify gray market cameras and deny warranty and repairs on them. In essence, Nikon’s trying to act like as series of local companies in a global environment.