1920 Melbourne Halfpenny
Some older Australian coin catalogues make mention of a pattern 1920 bronze Melbourne halfpenny (p64, Skinner, Renniks Australian Coin and Banknote Values, 1989). The records themselves are unclear - Mullett certainly mentions the Sydney Mint striking 1920-dated halfpennies, though he doesn't state the source of the dies or whether the Melbourne Mint struck any (p3, Mullett, Australian Coinage An Account of Particular Coins, 1991). According to Sharples, the Sydney Mint received its 1920 halfpenny dies direct from London (p14, Sharples, Australian Coins 1919 to 1924 in Journal of the Numismatic Association of Australia, July 1985). Interestingly, he also notes that in preparation for the striking of 1921 halfpennies, the Sydney Mint sent a pair of specimen halfpenny dies to the Melbourne Mint for guidance on the production of master dies. This was done while the Sydney Mint had 1921 halfpenny dies on order from the Royal Mint in London (p14-15, Sharples, Australian Coins 1919 to 1924 in Journal of the Numismatic Association of Australia, July 1985), so presumably the specimen dies sent were dated 1920. This situation could explain the existence of a 1920 Melbourne halfpenny. While the Museum Victoria collection (incorporating the Melbourne Mint collection) has no examples, this is probably not surprising as coins would have had no special significance, and were struck in 1921 with 1920-dated dies - merely test coins produced from obsolete dies.
While the halfpennies dies used at the Sydney and Melbourne Mints were identical, the usage of dies from the Sydney Mint would make identification of such patterns even more difficult. An example was reportedly sold in Noble Numismatics' July 1994 auction as lot 1484. It is not photographed and presumably the provenance is the only proof that the coin was struck in Melbourne and not Sydney.