/ubbthreads/images/graemlins/hammer.gif Let us not overlook the very likely possibility that "some previous owner did it." In this case, the "it" is remove the rear reflector pods since the rear end of these cars looks better without them. It is not an uncommon decision to remove these pods during a restoration or even just a paint job.
My understanding is that these reflectors were added due to a new legal requirement in the UK. Therefore it is not likely that Austin would have been haphazard about installing them. I would doubt the proposition that they just skipped putting them on some cars because they happened to be out of stock or something. Much more likely, in my opinion, is that if they are not present on a car made after the date when they apparently started appearing, is that it is due to a previous owner removing them.
You have to be pretty careful about drawing conclusions from a sample of one car. For example, I'm currently in correspondence with a Bugeye owner who asserts that his Bugeye came with a boot lid. I would suggest that yes indeed it came TO HIM with the boot lid present, but it did not come from the factory that way.
The "DPO" (Dreaded Previous Owner) was/is capable of any number of atrocities (aka "modifications"). In my experience, anomolies are much more likely due to a DPO than to The Austin Motor Company.