As far as I can tell, you can determine what's inside an Xbox from the 8th and 9th digits of the serial number. That's the first two digits of the 5-digit part.
This means that you can tell what's inside the Xbox whilst still at the shop, by reading the serial number through the hole in the packaging!
Here's what I've found so far:-
21 Thomson DVD, Western Digital HD, 1.0 PCB, PAL
23 Philips DVD, Seagate HD, 1.1 PCB, PAL
24 Samsung DVD, Seagate HD, 1.1 PCB, PAL
30 Philips DVD, Seagate HD, 1.2 PCB, PAL
More to come as I come across them (spotted a 20 but didn't note its contents).
Any additions (especially NTSC ones)/exceptions/comments gratefully received!