PS2 Slim model number query

[Ryu Kazama]

I've got a Japanese PS2 slim that happens to have a model number of....SCPH-*****

What exactly does that mean? There's no usual sticker on the console itself or even the Warranty sticker yet I believe this has never been opened. Does anyone happen to know what this could mean? Usually one refers to model numbers as 'SCPH-XXX' or variants, where the 'X' is a number itself. But this simply *****. So unusual.

Pic attached.