The pair of unique number plates sold at auction come with an interesting history.
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The South Australian number plate '1' has sold at auction for $700,000.

Pickles Auctions offered the number plate alongside '2', which ended up selling for $185,000.

But while '2' may have been a 'relative' bargain, website historicplates.com.au says there's more than one South Australian '1' number plate in existence.

When the very first number plates were issued in the state, South Australian authorities retained the rights – meaning the rights to display the plates cannot be transferred between individuals, and must be forfeited to the government. These are known as 'SA1'.

In 1985, to celebrate the Adelaide Grand Prix, a limited number of 'Grand Prix' number plates were sold at auction – together with the transfer rights to those plates. These are referred to as 'GP1'.

It's understood those who have 'SA1' on their vehicle are aware of the clause, and have kept the number plates in use for decades to avoid having to hand them back to authorities.

This means there could be two vehicles registered in South Australia with the number plates '1'.

While the $700,000 sum for 'GP1' is the same as the median price for a three-bedroom house located in Adelaide according to Domain, it falls well short of the value of Victoria and NSW's '1' number plates.

According to numberplates.com.au, Victoria's '1' is owned by the former Foster's Group CEO (later Carlton & United Breweries), who is said to have negotiated the plate as part of his severance package back in 1992.

The plate is often seen around Melbourne, registered to a Mercedes-AMG wagon, and is estimated to be valued at more than $2.5 million.

Though less is known about NSW '1', in late 2017 the NSW plate '4' sold for $2.45 million at auction.

Our thanks to historicplates.com.au and numberplates.com.au for much of the background information contained within this story.