There was probably a higher than normal need for replacement axles around this time based on the track and street racing that was taking place. COPOs, L78s, etc. And the Novas used the same axle, so that added to the need as well.
A couple of possible answers to the question that Bullitt raised about 'no stamping' on the axle... besides oversight.. is this:
1) Did replacement differentials (bought OTC) have the same date stampings? (I'm guessing they probably did if they were complete rears)..
2) BUT... a replacement housing (no gears/axles/etc) would probably NOT be stamped, since it was sold without gears, etc. I suspect this was what happened to the car that Bullitt questioned...
Note: With the '70 models changing axle length, isn't it 'likely' that GM parts depots would stock spares for the '67/8/9 differentials?? and the cast dates on those 'spare parts' would more than likely occur near the end of the production year.. and August '69 would qualify for such a period?