Pyrs can have the double dew claws, too, so that wouldn't necessarily be a dead giveaway. I do think that this is a mix, and my guess is Maremma x Aussie, too.
A Maremma would have less angulation in the hock than an Aussie, something like a Pyr, and would not have as straight a topline. An Aussie should have an almond shaped eye, not a round eye. Stockdogs would be smaller - my girl is not even 19" and about 35 pounds. Her daddy was 40 pounds, and not much taller. It's correct that they are deliberately smaller to keep out of the way of cattle kicks - heelers go in for the grab sideways and need to stay under the kicks. Perfect example on this page: http://users.htcomp.net/slashv/home.htm
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