I know from memory that Holden did have a "preferred buyer" scheme going in the mid 70's where those who frequently upgraded their vehicle (ie, every couple of years or whenever a new model was released) were able to order options that were not always widely available to the public on certain vehicles. Some of my relatives used to do as such, one of them ordering a '75 HJ Kingswood ute with just about "the works". The only thing it didn't have was GTS guards.

Maybe the original poster's carpenter mate was the son of or knew someone on the inside and qualified as a preferred buyer (or whatever they called it)?

Regards,

Dave