One question. In all your years of observations, have you ever seen the year on this type of dater represented by two digits?
Remember, these marks simply indicate the date, as set on that particular stamper, that some document was received by a postal worker, or looked at, or "it was in this box so I stamped it".
It's not like these dates are a critical component in the creation or evaluation of a precise flow of documents.
Think of it this way, the big tsunami last year came in and swept the shoreline pulling in a huge mass of debris. That debris is wending its way across the Pacific. It will arrive here eventually but some of it will go back, and yet other parts of it have already landed and been scavenged. Still, we know the data that the debris mass was recognized and people went out there and dropped marker buoys all over the place. They flow with the mass.
The buoys, however, DID NOT CREATE THE DEBRIS. Likewise a ROUND DATE did not create the document its on.