The King's Norton Metal Company Birmingham (KN) minted many British copper and bronze coins over a period of years. No British coins were minted from 1920 onwards, but they continued to mint coins for other countries.
There were only 7 or 8 British 1933 Pennies minted by the Royal Mint London and none of them were released into circulation.
There is no mintmark on the 1921 British Penny, they were all minted at the Royal Mint London.
All British Pennies from 1883 to 1911 inclusive have no mintmark. They were all minted at the Royal Mint London.
There are three variant mintings of the 1918 and 1919 British Penny. The pennies minted in London have no mintmark. The mintmarks "H" for the Heaton Mint, Birmingham, or the "KN" for King's Norton Metal Company are to the left of the date on the reverse and are extremely small.
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.
I presume that you are referring to a circulated US 1931 cent with no mintmark under the date, rather than a British penny. The answer then is almost certainly under a dollar.
British 1875 Pennies were minted at either the Royal Mint London, or the Heaton Mint in Birmingham. The Royal Mint Pennies have no mintmark. The Pennies minted at Heaton have a small "H" below the date on the reverse. All British Pennies minted at Heaton from 1874 to 1882 inclusive will have the same mintmark in the same place.
A mintmark is a small identification mark used to tell which mint it was made in.
underneath the year
It will have a S mintmark.
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 English/British One Penny coin has been issued for over 1,100 years. All years of issue of the British decimal New Penny and Penny are still potentially in circulation so, unless they are part of a Proof or uncirculated mint set or are individual Proof or uncirculated coins and in absolute mint condition, they are worth 1 Penny. 20th century predecimal Pennies would need to be in mint condition to attract any significant value. The most spectacularly valuable example would be the 1933 Penny. There are 7 known coins. Other than that, there would be the - 1919 (KN mintmark & H mintmark), 1918 (KN mintmark & H mintmark), 1909 (variant - 1 directly above tooth) 1902 (variant - with low tide - further done Britannia's shin)
Philadelphia. Cents (pennies) from the Philadelphia mint have never carried a mintmark.