I have been following this thread because I also do not have the segment of ownership from the original sale of my 67 Mustang in Barstow California till my purchase in early 1978. Usually, California cars retain their plates unless they become damaged or destroyed for certain reasons. My original plates and any previous ownership was lost because the car had been involved in a somewhat minor accident and at 10 years old, easier to TOTAL the car than repair it (amazing, I fixed it with only $120 in the day) The Marti Report gave me some info and I've tried (also state of California) the DMV and selling dealership (which has just recently changed owners again) with no avail. I imagine that piece of history will remain as unsolved unless a contact at or through Ford Motor Company releases any more than what Kevin Marti has access to, nothing else likely can happen.
In 1984, I purchased a set of "California Sunset" vanity plates for my car, replacing the Blue/Gold plates that were issued in 1978 (from memory, plate number 967 VGU) The "Sunset" plates were one of 4 styles available that year and to me looked great! I had a difficult time looking through a book of previous plates that were unavailable but finally found one I liked. I wanted the year (67) and GT or GTA in the plate and ALL of my ideas I wrote down before going to the DMV had already been taken. I DID manage to secure 67GT4ME as a plate number.
I say everything above because it lays the groundwork for what I will be using as plates on my car when I show it. A couple years back, I found a site that made custom plates for all states and countries, for all years. They claim to make plates for movie props. Anyways, My wife bought me a set of BLACK/GOLD California license plates with my 67GT4ME plate numbers/letters on them and I swear, they look just as original and NEW as in 1967. I did have to buy a validation sticker from another source, but I plan on using these when I show the car. I could have ordered ANY comination of letters, numbers I wished but instead used the ones I had made by the state of Ca. in 1984. I live in OHIO now, so I cannot actually drive on these legally, but I have found a way I can attach the Ohio 67 YOM (year of manufacture) plates over the top with plastic wing nuts & rubber washers from the backside, ...that is should I ever have a need to drive it on the roadways.
I looked for the site again this morning, but I see they are no longer selling the California plates (maybe State of California blocked or sued them, I don't know) but there is a "Special Request" form you can attempt to have a set made with. Given that you are in Australia, they might be willing to do them. It won't hurt to try. When my wife ordered these a few years ago, the site mentioned they would NOT ship them to California, I was in Ohio, so I got them. I imagine once again, the unscrupulous took this avenue to order them directly from a web order anyways.
Again, at that time, we COULD HAVE ORDERED WHATEVER ARRANGEMENT WE CHOSE, there were no restrictions I was aware of. This to me is saying to anyone reading this comment, you can order an original plate with BOSS302 or whatever you want, even if somebody else already has that arrangement in your state, you just cannot DRIVE on them! I am OK with that, I will have a SHOW car, not a driver car and as I said, I won't be stupid and drive on them anyways simply because "Who will know..."...I'll know, and heaven forbid if needed, I am sure the insurance company would know if I ever needed them "by accident".
For anybody (outside California) who simply needs a really nice set of original looking show car plates, here is a link that can get you to the correct page for that.
http://www.licenseplates.tv/special-request-form.phpRichard