There's no such thing as an FX Holden
It is believed that Repco were responsible for the FX prefix. Parts books in 1957 show the first reference to the FX Holden and this seems to have come about by Holden naming it's second vehicle the FJ and than third variant the FE. So why FX? Well the "F" is easy to figure but the "X"? The letter X is used because X typically stands for unknown! So is it true? Who knows! but it sure makes a good story.