Old head half sovereign obverses

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Indeed. The number of these coins left though is pretty small though right?

What was the story with die production in Australia at the time? Were they sent from England or did at least one of the local mints have the kit to make them here?

Re: survival rates I cant comment except to say that half sovs would probably have the lowest survival rates out of all pre-decimal coins.
Re: die production - My understanding is that working dies were produced at the Australian mints from Masters sent from England.

From what I've researched, the working dies were produced at London too up until at least 1880, after which the Melbourne mint produced the working dies for Sydney and Melbourne. I don't know how long this continued for but the Perth mint produced all of their own working dies.