Yeah, what roger said!

The subdir names are very simple: _xxxxx
where "xxxxx" are the most significant 5 digits (hex) of the 8-digit original "fid" (file identifier). The files within each subdir are then named using the remaining 3 least significant digits. IF the original fid file had fewer than 8 digits, it must first be padded with zeros on the left to get an 8-digit value to then do the 5:3 split on.

Cheers