It looks like when someone orders from Japan, the address comes through on the receipt with a 2-digit number in place of the prefecture. For example, a customer who ordered something yesterday put in a billing address in Norway and a shipping address in Japan, and what showed on the receipt was (name and street addresses changed for privacy):
123 Fake St
C/O Jane Doe
123 Madeup St
For the Japan address, the customer entered "Tsu-shi" for the city, "Mie" for the prefecture, and "5140821" for the postal code. On the receipt, "Mie" appears in the middle of the Norway address, where it does not belong, and "24" appears in its place in the Japan address. From some looking on wikipedia, it seems like there might be a numbering system that links 24 and Mie prefecture, but that's the only place I see the link. In all the Japanese address guides I've found, it's always the name of the prefecture, not a number.
Apparently our packages have been getting to Japan just fine, so it's not an urgent issue, but we'd love it if someone could look into this and see if there are some wires crossed somewhere! We are not using the automatic postal code lookup, by the way.