There's a post awaiting addition to the FAQ, but in the meantime, here's what I've found:-
On PAL Xboxes, the guts of the unit can be determined from the 8th and 9th digits of the serial number, in other words the first 2 digits of the 5-digit section (that's right, no need to open the box!)
Codes found so far are:-
21 - Thomson DVD, Western Digital HD, 1.0 PCB
23 - Philips DVD, Seagate HD, 1.1 PCB
24 - Samsung DVD, Seagate HD, 1.1 PCB
30 - Philips DVD, Seagate HD, 1.2 PCB
More codes to come as I come across them!