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Whilst the mint does indeed use heptagonally pre-shaped blanks for striking 20p's and 50p's, for the dodecahedral brass threepences and now the new pounds, they use circular blanks (the external angles are more shallow and can be squeezed ok into the 12 sided collars that are used). Proof of this is both during WW2 when steel was at a premium, the mint used "blunter" cornered collars which lasted longer than the normal sharp cornered ones, leading to slightly smoother angled coins, and also with this 1960 threepence brockage which has been struck proud of its collar and retains its circular shape... It weighs 6.85 grams too, so I am perfectly happy your piece is a legitimate blank. The rough surfaces and edges are no problem either - they are all squeezed out smooth when the annealed blank goes through the press. Other denomination blanks I have are similarly a bit rough and ready!

I've come across what I believe to be a blank for a nickel brass threepence. It weighs 6.84 grams which fits, but I've never come across one before as I imagine they'd have been hard to smuggle out of the Mint. (I also wasn't expecting it to be circular as I'd never thought about how the geometric shape would've later been made!). Has anyone else come across something similar?

Thanks for your replies, everyone! I'll do some more digging. I would assume the Royal Australian Mint, judging by John's alias.

Nice one Gary. I knew you'd have one. 🙂Did it take long to find? I've been searching 6 years plus. 475's I've found 6. 473's. I've found 3. R3 and R5.

1956 4 & c obverse

1956 4 & c reverse

Not the best macro shots from my phone, but a doable portrait…for me at least

Here you go. Both different dies. My reverse is over ermine as well.