I've seen a few listings of CDA's and what their customisation is. None
that mention 162004/29 though.

My understanding is that 162004/29 is an Australian generic P900 with no
branding or locking to any carrier.

However, S/E hasn't released any recent firmware upgrades via the online
upgrade site for that CDA. The latest is R3A06. Apparently it's claimed
that the fixes in later firmware don't apply to that CDA version.
(Personally I think this is bumkin - a new version number such as R4A or
R5B means new features and general bugfixes that apply to the range of
phones, not just one particular CDA type)

Does anyone know if a later software version than R3A06 is available
through a service centre for this CDA? Is there a time when Sony
Ericsson will update the available software for this CDA or will it
always remain R3A06 while R5B02 is available for most other phones?

Online updating is useless when you can't update to the latest version...