Since the population of Carpet Pythons is (and by all accounts, DNA and otherwise, we can assume always has been) continuous, with gene flow along the east coast from Victoria to Cape York, how is any area more pure than any other area? Why is a Sydney Carpet ('Diamond') not an intergrade between a Victorian Carpet ('Diamond') and a Gosford Carpet ('Diamond')?
Why are the 'Moist Forest Carpets' ('intergrades') like the stunning Kempsey animals not pure, while Brisbane Carpets are intergrades between those and Prosperpines?
Sydney Carpets are just as much intergrades as 'intergrades'. Intergrades are just as pure as a Sydney 'Diamond'.
How can it make sense to call something at the northern end of the supposed 'pure Diamond' range (a bit north of Newcastle, depending on who you ask) a 'pure' Diamond when they are clearly intermediate between what you find in Sydney and what you find in the 'middle of the intergrade zone'?
Perhaps people are comfortable calling something a 'pure diamond' if it has spots/'rosettes' rather than stripes and blotches, but of course that's just one of many visual traits they have, and there are exceptions deep in the 'pure Diamond' zone (though I've never come across an exception deep in the so-called 'pure carpet' zone. Then again, Diamonds always have at least a little bit of lateral striping, so even that arbitrary diagnostic tool breaks down.
At any point along the east coast (with some exceptions in north QLD where things get a bit funny in some places), every population of Carpet Python is quite variable, and generally intermediate between the Carpets north and south of that area. The Carpet Python populations along that range are not more or less pure. If we had established a big colony at, say, Kempsey rather than Botany Bay and Sydney was further north, with Brisbane still where it is, the original Carpets collected would have been from Kempsey rather than Sydney and they would have been the type we first named, and described. If Brisbane was where Proserpine is, we'd have a very different concept of what typical 'Coastal Carpets' look like (and because that's where the big population was, that's where most of the captive stock would have originated).
I think people are still stuck in the myth that there were once 'pure Diamonds' and 'Pure Coastal Carpets' and nothing in between, then at some point they started interbreeding to make 'intergrades'. If that was the case they would be hybrids, but they're not. This is just not the way it happened (and that's one thing the mtDNA does indicate to us). Things make a lot more sense and are far easier to understand once you realise that it's all one big continuous population and always has been. Imagine it as one big spectrum with white at the NSW/Vic border and black at the tip of Cape York, a gradual grey scale in between, and people drawing random lines along the coast trying to split up various areas (with animals and their genes flowing all along, completely ignoring any lines we might decide to draw).
I'm simplifying a little, but that's the basic way it works, not the 'there were once two types which later made hybrids' myth.