The 1922 Australian Penny has no mintmark. Actual mintages were Melbourne - 9,529,440 and Perth - 3,168,000. Identical dies were used so the issuing mint for a particular 1922 Penny cannot be determined.
An Australian 1942 Halfpenny minted in Melbourne, has no mintmark. An Australian 1942 Halfpenny minted in Perth has a dot (.) after the (Y) in PENNY (PENNY.). An Australian 1942 Halfpenny minted in Bombay has an (I) below the King's head.
An Australian Penny or Halfpenny with a "Y." (PENNY.) mintmark indicates that the coin was minted at the Perth Mint. An Australian Penny or Halfpenny with a "I" mintmark indicates that the coin was minted at either the Calcutta or Bombay Mints in India.
An Australian 1956 Penny looks almost exactly the same as any other Australian Penny minted from 1955 to 1964 inclusive. A 1956 Penny minted in Perth will have a mintmark (.) after the "Y" in PENNY (Y.). The Melbourne Penny has no mintmark. See the 1964 penny at the link below.
1956 Australian Pennies minted at the Melbourne Mint have no mintmark. 1956 Australian Pennies minted at the Perth Mint have a dot (.) after the Y in PENNY (PENNY.)
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).
KG is not a mintmark, it is the initials of the designer of the (kangaroo) reverse of the Australian 1938 to 1964 Penny and Halfpenny coins, George Kruger Gray. 1940 and 1941 Pennies minted at the Perth mint have a dot between the K and the G (K.G) as the mintmark.
An Australian 1948 bronze Penny (George VI)(minted in Melbourne - no mintmark), uncirculated and in absolute mint condition could fetch up to $48 AUD. If it has been circulated but still in good condition, it might fetch anything from $0.75 to $12 AUD. There were 26.616 million minted. An Australian 1948 bronze Penny (George VI)(minted in Perth - mintmark = Y.), uncirculated and in absolute mint condition could fetch up to $1,250 AUD. If it has been circulated but still in good condition, it might fetch anything from $2.50 to $150 AUD. There were 1.533 million minted. An Australian 1948 bronze Penny (George VI)(minted in Perth - mintmark = Y.)(Proof), uncirculated and in absolute mint condition could fetch up to $50,000 AUD. A reputable coin dealer will be able to give a more accurate valuation.
The 1944 Australian Penny was minted at one of two mints. Melbourne Mint (no mintmark - 2.45 million minted. Perth Mint (mintmark = Y.) - 27.83 million minted.
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.
Australian Pennies from 1955 to 1964 inclusive were minted at either or both of the Melbourne Mint and the Perth Mint. Pennies minted at the Melbourne Mint have no mintmark. 1956 Pennies minted at the Perth Mint can be identified by a dot "." after the "Y" in PENNY, eg. "PENNY.".
Other than mintmarks and the year, the Australian 1948 is almost identical to all Australian Pennies minted from 1938 to 1948 inclusive. The legend around the circumference on the obverse reads "GEORGIVS VI D:G:BR:OMN:REX F:D:IND:IMP." in heavily abbreviated Latin meaning, "George VI, by the grace of god, king of all the British territories, defender of the faith, Emperor of India". The letters "HP" below the bust are the initials of the designer, Thomas Humphrey Paget. The reverse has "AUSTRALIA" around the circumference at the top, and "PENNY" around the circumference at the bottom. There is a left facing hopping kangaroo centrally located with the Commonwealth Star below the kangaroos head and the date is below the kangaroos tail. The letters "KG" below the kangaroos tail are the initials of the designer, George Kruger Gray. 1948 Pennies minted in Melbourne have no mintmark. 1948 Pennies minted in Perth have a "." after "Y" in PENNY (PENNY.) as a mintmark.
An Australian 1953 Penny minted in Perth has a dot (.) after the (A) in AUSTRALIA (AUSTRALIA.). All other Pennies were minted in Melbourne and have no mintmark.