A mintmark is a small identification mark used to tell which mint it was made in.
There is no mintmark on the 1921 British Penny, they were all minted at the Royal Mint London.
It will have a S mintmark.
underneath the year
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.
Philadelphia. Cents (pennies) from the Philadelphia mint have never carried a mintmark.
A 1943 steel penny with no mintmark can be worth $.35 - $1.50
If it has one , it's under the date
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.
A 1941 penny without a mintmark means it was minted in Philidelphia.
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.
All British Pennies from 1883 to 1911 inclusive have no mintmark. They were all minted at the Royal Mint London.
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.