G'day Reece. I'm no expert, but I think generally around 8 - 12 weeks before the build date on the tags, is normal unless there was some factory delay.
Just check you are coding the engine correctly, or put the numbers & tag info up here for someone to advise.
Alternately ask Byron on fastlane.com.au he should be able to give advice if it lines up with other cars built at same time.
Ben at H.H.S. (Holden Historical) has holden production documents, & will give you certification for around $90

Good luck with the purchase
Max