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.)
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.
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.
The Lincoln Penny was produced in both Philadelphia and Denver in 1956 (the latter have a "D" mintmark below the date).
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.
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.
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.
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.
HP is the initials of the designer (Thomas Humphrey Paget) of the obverse (heads) side of the coin. This will appear on all Australian Pennies from 1937 to 1952. KG is the initials of the designer (George Kruger Gray) of the reverse (tails) side of the coin. This will appear on all Australian Pennies from 1937 to 1964. PL is a mintmark and indicates that coin was minted at the Royal Mint London. The 1951 Penny was the only Penny of this design to be minted in London. A dot after the "Y" in PENNY (PENNY.) is the mintmark indicating that the coin was minted at the Perth Mint. No mintmark indicates that the coin was minted at the Melbourne Mint.