The compression type field identifies "17480" as "DRMed", but this is not necessarily the case. 17480 means HUFF/CDIC compression is used. The encryption type (at total header offset 0x0C) is the correct indication of DRM, as mentioned in the second table.
There seems to be some uncertainty about the location and size of the four DRM fields in the MOBI header. I've just noticed that the edit I just made is probably the reverse of the previous edit. I was following mobi/writer.py in the Calibre source code, which has each of those fields with 32 bits. The Calibre source code also explains some other fields that are currently undocumented in this wiki. Edmundo 03:40, 6 February 2011 (EST)
For statistics on this page see: Page Statistics#6 MOBI
I find EXTH records of codes 450, 451, 452 and 453 in some amazon ebooks I got recently. Their contents seem four byte binary values. 452s hold 00000000 in all cases I saw. No idea what they are for. :-(
 Hex View
Can someone more knowledgeable make this article more confirming to real life ? I looked at my .mobi books with hex-viewer but its header doesn't confirms with what I read here.