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.
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.
The year is at the bottom on the reverse side.
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.
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.
There are no rare 1912 British Pennies. The "H" on the reverse, below Britannia and to the left of the year, indicates that it was minted at Heaton in Birmingham. There were 16.8 million minted.
what is the value of a 1912 lincoln wheat penny
underneath the year
It will have a S mintmark.
A British 1912 bronze Penny (George V)(minted at Heaton - mintmark = H), uncirculated and in absolute mint condition could fetch up to £250 GBP. If it has been circulated but still in good condition, it might fetch anything from £10 to £90 GBP. A British 1919 bronze Penny (George V)(minted at Heaton - mintmark = H), uncirculated and in absolute mint condition could fetch up to £800 GBP. If it has been circulated but still in good condition, it might fetch anything from £15 to £250 GBP. The values quoted are the best possible for the best specimens in their grades, but may be significantly lower due to varying demand for the coin and the prevailing economic climate. A reputable coin dealer will be able to give a more accurate valuation based on inspection of the coin.