You'd be grasping at straws to get it close to right, as it is different across each plant, and the numbers used to track cars on each assembly line varies: Pagewood used the firewall number, Dandenong and Acaia Ridge used the PSN and Elizabeth used the build sequence number (BODY number on HQ-late HX, PSN on HZ). Add to this the fact that a lot of the cars we are interested in here (vans) were assembled at Pagewood an Dandenong using Elizabeth bodies with Elizabeth body plate, so imagine the confusion that threw up as there is no Pagewood firewall number and no Dandenong PSN for tracking. ANd then engines get thrown in at random. And chassis numbers get put on at different stages in each plant. Cras get pushed off to one side to get finished later if something is wrong. Therefore no real trend that any of us could come up with really helps. We can give possible the first 2 digits given a chassis number, but even then they can jump on consecutive cars from QL91xxxx to QT87xxxx to QR89xxxx then back to QL87xxxx. So all you can do is estimate and look close as you can. Engine number is what matters though, as long as the cast date is prior to your build then all is sweet.