The stocking of parts for production line assembly was not done on a JIT (just in time) basis when these cars were being built, that process came later. Holden often surplussed production line stock when they changed models. Their spare parts (HSPO) operated separately from the assembly plants as they ran two lots of purchasing teams. Their HSPO quality department at Dandenong often accepted supplier changed product compared to the quality processes today. HSPO continually purchased stock based on sales and only ran out when it was deemed not viable, or they superseded two parts into one because they looked similar or did the same job. Some changes were done because the tooling for a part was modified slightly to suit the next model. An example would be HQ consoles and HJ ones where they used the same tool for the lower floor section and a new top for the HJ. That is why the grains are different on the HJ ones.
As there were NO computers for stock records in those days Holden and their dealers often over-ordered, over-stocked and even ordered the wrong part which they couldn't return.It was this abundance of surplus stock that enabled Rare Spares and many others to purchase it, store it and sell it at a later date.