Probably more likely HJ pilot production was happening around 5/74, so the running change was made to HQ at the time. You see the same sort of thing with all series change, with early introduction of next series changes eg HJ trim and paint in HQ, RTS geometry in late HX, HX trim in late HJ etc. By around 5th or 6th month 1974 probably only Acacia and Elizabeth were building vans anyway. I've never recorded a Dandenong HQ van (which would have used Elizabeth bodies anyway), and Pagewood should have just about stopped van body building by May-June (they used Elizabeth bodies for HJ).