Excuse me 0037 but

http://world2ch.org/wiki/doku.php?id=chan_filenames

>4chan, 2chan, and all the others use unix timestamps for file names with three additional numbers tacked onto the end in order to avoid collisions

While you're mostly right, I'm pretty sure the last 3 numbers in 2chan are actually milliseconds! I'm not sure about 4chan, but Wakaba and Kusaba do generate those numbers randomly.

The more you know!