Australia Penny Error 1919 One Side And 1935 On The Other?

Magician's coin. Must be a seam somewhere.

Sorry, but it is obvious in this case that it cannot be a genuine "mint error". The mint would not have a sixteen year old die just sitting around waiting to be accidentally used instead of an obverse.

These coins are made by first taking two perfectly normal pennies. One penny is ground down flat on one side until it is very thin. The second coin is put into a lathe and hollowed out, making a shallow "bowl" which, if they've done their job properly, should just fit the thin disc of the first coin in nicely. You can see some examples of American and British coins treated this way in this thread. You can see the "seam", where the two coins are joined together, on the George V side of the British example in that thread. I believe I can see this seam on the 1935 side of your coin.

Sap, nothing is obvious about old dies sitting around. These pennies came from Melbourne mint, the same mint struck dies in 1920 that were sent to another mint (Sydney) that let some sit around until 1924 (the 1924 Indians) and then when Sydney closed one of those dies was sent back to Melbourne to be used on the 1927 Indian. Then there are restrikes of various coins with dies many decades old which are sometimes muled with dies from a different era.
But yeh, this particular mule is fake.