As I understand it, it's a tuple of lists of cubes, i.e. a slot stores a head of a linked list of cubes.
usbrip works with non-modified structure of system log files only, so, unfortunately, it won't be able to parse USB history if you change the format of syslogs (with syslog-ng or rsyslog, for example). That's why the timestamps of "Connected" and "Disconnected" fields don't have the year, by the way. Keep that in mind.