Unfortunately I don't have access to my dads records anymore, but he was a taxi owner.
I do remember that his paperwork referred to his 48-215 as "48 Holden sedan" which was then replaced in 1957 with an "FJ Holden", then in 1961 with an "EK Holden", followed in 1965 by a "HD Holden" (the first auto trans cab he owned), then followed by a "HQ Premier" (1970), with the last one a "HX Premier" (date unsure)
As you can see he would replace the cars on an approximate 4 year cycle. They usually received a "Short motor" (all bar new heads)after two years They would have close to 300,000 miles (yes, miles) on the clock when sold.
He would also replace the cable clutch on the early manual transmission models with a Hydraulic after market system by the time they were 6 months old!
The coding would have to have changed in late 1963 when the EH was released, because the codes to follow were HD, HR, HK, HT, HG, HQ, HJ, HX, HZ.
Holden had a habit of releasing their models in October or November of the previous year (EH - 1964 model was released in late 1963), likewise the 1965 HD was released late 1964, etc etc)
A comprehensive list can be found here http://en.wikipedia.org/wiki/List_of_Holden_vehicles_by_series