Sorry; only just read this today. [I've got a D4 now].
The answer's simple although probably already deduced
The Navigation firmware is on the Navigation DVD and not on the MMI update CDs. If you swap an old DVD back in the Navigation drive and run a VCDS, you'll see the firmware level of the unit is backleveled ...
Julian
Old mechanism:
Address 37: Navigation Labels: 4E0-910-88x-37.lbl
Part No
SW: 4E0 910 887 D HW: 4E0 919 887 C
Component: MNS ECE H43
0550 [Firmware level]
Revision: 00000000 Serial number: 353EJ047522731
Shop #: WSC 02313 785 00200
VCID: 2E05097BDC17BBEBD1E-807A
No fault code found.
New mechanism:
Address 37: Navigation Labels: 4E0-910-88x-37.lbl
Part No SW:
4E0 910 888 P HW: 4E0 919 887 C
Component: MNS ECE H43
1060 [Firmware Level]
Revision: 00000000 Serial number: 353EJ047522731
Shop #: WSC 02313 785 00200
VCID: 3B23302F1F655E439A1-806E
No fault code found.