Thanks everyone who has provided info on their scope serial numbers. I'm pretty sure HBMCo had stopped dating them by the time they got to 3100, so if I'm right we're looking at a hundred dated scopes at most, & quite possibly rather less. We know 3057 was dated, so the change to omitting the date must have occurred somewhere between 3058 & 3100, assuming the application or omission of the date was done consistently.
If anyone knows of any HBMCo scopes in the serial range 3058 to 3100 or thereabouts, details would help us narrow down the changeover somewhat. It's only a minor detail in the 4T story, but it's just a little itch I'd love to successfully scratch!