There will be an Engineering Change Request for it, which will have the reason and the cost saving for it detailed. As Innuendo says, it doesn't really matter why it happened, it just did. When it happened will vary across the 3 x body plants making HQ bodies (Pagewood, Elizabeth and Acacia Ridge) and also the 4 x plants assembling them (same as body plants plus Dandenong). Dandenong probably changed a bit later (in ADR plate terms) than the others as it was using Elizabeth bodies, and there was a shipping (actually "training") time involved. Change would also depend upon the level of pressed panels at each body plant, and where the van assembly was in their respective rolling schedules. Woodville (SA) pressed all the panels and they'd be trained to each assembly plant depending upon their rolling schedule requirements. Then there is the cars that get held up for whatever reason. I've found a few cars that really should be for example January completion but aren't completed until March. Some of these will have got an ADR plate before the delay (like my Premier, 12/74 ADR but completed late in Feb '75), but some will have for example the body fall off the assembly line hooks before the ADR and/or PSN is assigned (eg Pagewood cars got their ADR plate and VIN plate right at the end of the line). It goes back to the body plant to be repaired when they get a chance. If this happened to be a HQ van at the Dandenong assembly plant, it'd still have its Elizabeth BODY tag which would align it with say 6/74 Elizabeth vans, but it may not get completed and get an ADR plate until 8/74 meaning it still has side gills but it is 8/74. Of course this will all be obvious by the combination of the tag numbers, but on first glance it might skew the evidence of when the gills disappeared a bit. The other example, being my HJ Prem, failed something after the line, so maybe the water test (eg windscreen leaked) or there was a mechanical problem etc, so it got pushed aside waiting for the appropriate time to fix it eg an assembly plant outage.