I am unfamiliar with the reference to "centre diamond" on an Australian George V Penny. Please provide more information.
George Penny was born 20 May 1845
The last Australian Penny was minted in 1964.
The last Australian Penny was minted in 1964.
The last Australian Penny was minted in 1964.
The last Australian Penny was minted in 1964.
The Australian 1943 Penny was minted variously at the Melbourne Mint (no mintmark), Perth Mint (mintmark is a dot after the Y in PENNY "Y.") and Bombay Mint (mintmark is an "I" under King George's bust).
There is no "MP" mintmark on any Australian Penny. A 1941 Australian Penny minted in Perth will have a "." after the "Y" in PENNY (Y.), or a "." between the KG (K.G) on the reverse. A 1941 Australian Penny minted in Melbourne will have no mint mark at all. The letters HP on the obverse below King Georges head are the initials of the designer, Thomas Humphrey Paget. The letters KG on the reverse above the kangaroos tail are the initials of the designer, George Kruger Gray.
The last Australian Penny was minted in 1964.
The last Australian Penny was minted in 1964. There was never a 1975 Australian Penny.
The Australian Penny was a coin used in the Commonwealth of Australia prior to decimalization.
Such a coin does not exist. The first Australian Penny was minted in 1911.
Australian Pennies have never had a Roman soldier on them. You are describing a British Penny, and the "Roman soldier" is the goddess Britannia who personifies Britain. Britannia has appeared on various British coins for hundreds of years.