Philadelphia. Cents (pennies) from the Philadelphia mint have never carried a mintmark.
There is no mintmark on the 1921 British Penny, 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.
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.
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 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).
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.)
Australian 1920 pennies were minted in both Melbourne and Sydney. On the reverse side underneath the word "PENNY", is a scroll. Coins minted in Melbourne will have a small dot under the scroll, coins minted in Sydney will have a small dot above the scroll. Some coins have no dot and could have been minted at either the Melbourne or Sydney Mints. The "no dots" coins are classified as rare. This means of determining the mint, only applies to 1920 pennies.
It means that the coin was minted at the San Fransisco mint. If a penny has a D, it means the coin was minted at the Denver mint. If it has no mintmark, it was minted at the Philadelphia mint.
A 1941 penny without a mintmark means it was minted in Philidelphia.
No mint mark indicates that the dime was minted in Philadelphia.
If there is a mintmark at all, it will be a very small "H" to the left of the date in the exergue. The "H" indicates it was minted at Heaton in Birmingham. All 1912 Pennies without a mintmark were minted at the Royal Mint.