There is no mintmark on the 1921 British Penny, they were all minted at the Royal Mint London.
underneath the year
If you refer to the British Five Pound (Crown) coin, they do not have a mintmark. They were all minted at the Royal Mint in Wales.
No Indian Head cent had a mintmark until 1908, so the 1898 cent will not have one.
A 1921 Lincoln cent is not considered rare or even scarce. But, pre-1934 Lincolns are getting a little hard to find.
See the link below.
The letter "D" under the date of a penny is the mintmark which indicates where the coin was minted. There are 4 operating mints in the USA. One , represented by the mintmark "D" is in Denver, Colorado. Another, represented by the mintmark "S" is in San Francisco, California. Another, represented by the mintmark "W" is in West Point, New York. The 4th, represented by the mintmark "P" is in Philadelphia, Pennsylvania. You will find these mintmarks on all coins struck in the USA today with the exception of the pennies which are produced at the Philadelphia mint. You will find no pennies with a mintmark "P". You will also find no coin minted in 1965, 1966 and 1967 with a mintmark. This is because during those years, to discourage coin collecting which some blamed for a coin shortage, none of the mint facilities in the US placed mintmarks on the coins they produced. This practice ended in 1967 and in 1968 mintmarks were again placed on US coins.
Pictures of a specific coin are difficult to come by. Usually you should be able to find one on eBay, but the picture is withdrawn after the coin is sold. A 1928 British Penny would be almost identical to the 1936 Penny shown. See the link below.
It would depend on what you wanted to find out, but see the link below for a good starting point.
Pictures of a specific coin are difficult to come by. Usually you should be able to find one on eBay, but the picture is withdrawn after the coin is sold. A 1921 British Penny would be near identical to all British Pennies minted from 1911 to 1922. See the link below for an almost identical 1920 Penny.
If the coin does have a mintmark it would be under the "DO" in dollar on the back of the coin.
Any 1942, 1943 or 1944 Australian Threepence, will have the "S" (San Francisco) or "D" (Denver) mintmark on the reverse of the coin, on the right hand side, just below the fourth digit of the year. Any coins minted in Melbourne will have no mintmark.
British Edward VII (1902-1910) Sovereigns and Half-Sovereigns were minted at the Royal Mint London, Ottawa, Melbourne, Perth and Sydney Mints. The coins minted in London have no mintmark. The Sovereign and Half-Sovereign coins minted elsewhere have a mintmark "on the ground" below the horse and above the date. The coins minted in Ottawa have a mintmark "C". The coins minted in Melbourne have a mintmark "M". The coins minted in Perth have a mintmark "P". The coins minted in Sydney have a mintmark "S". The letters BP to the right of the date is the designers initials (Benedetto Pistrucci) who designed the St George and the Dragon scene.