US mint set were minted for sale to collectors. It provided a defined set separate from the standard business strikes.
1983 to 1997
Australian Threepences minted from 1910 to 1915 were minted at the Royal Mint London and have no mintmark. Australian Threepences minted from 1916 to 1926 were minted at either the Sydney Mint or the Melbourne Mint. Sydney Mint coins have no mintmark, the coins minted at the Melbourne Mint and have an "M" below the date. Australian Threepences minted from 1927 to 1941 were minted at the Melbourne Mint and have no mintmark. Australian Threepences minted from 1942 to 1944 were minted at the Melbourne Mint, the San Francisco Mint (USA) and the Denver Mint (USA). The coins minted at the Melbourne Mint have no mintmark. Coins minted at the San Francisco Mint will have an "S" below the units digit of the date. Coins minted at the Denver Mint will have a "D" below the units digit of the date. Australian Threepences minted from 1947 to 1964 were minted at the Melbourne Mint and have no mintmark. Some Australian Threepences minted in 1951 were minted at the Royal Mint London. The mintmark is "PL". The "P" appears above the first "E" in "THREE", the "L" appears above the "E" in "PENCE".
A mint set contains uncirculated coins from a specific mint in a specific year. Proof coins are specially made for collectors, minted with a higher quality finish, even mirror-like on some surfaces. Currently in the U.S., mint sets are available from Philadelphia and Denver, while proof coins are minted in San Francisco.
Dream..........on
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.
Australian coins minted at the Melbourne Mint do not have a mintmark.
In 1942, At the Philadelphia mint there were 657,828,600 minted for circulation, and 32,600 proof pennies minted. At the Denver mint there were 206,698,000 minted. At the San Francisco mint there were 85,590,000 minted.
British 1875 Farthings minted at the Royal Mint (no mintmark) - 712,760 minted. British 1875 Farthings minted at the Heaton Mint (mintmark = H) - 6,092,800 minted.
of Mint
From its initial issue in 1910, until 1915 inclusive, the Australian Threepence was minted at the Royal Mint London and has no mintmark. From 1916 to 1926 inclusive, the Australian Threepence was minted at either the Royal Australian Mint Melbourne or Sydney. Coins minted at the Melbourne Mint have a mintmark "M", coins minted at the Sydney have no mintmark. With a few exceptions, from 1927 to the last minting in 1964, the Australian Threepence was minted at the Royal Australian Mint Melbourne. Coins minted at the Melbourne Mint have a mintmark "M" if they have a mintmark at all. From 1942 until 1951 inclusive, Australian Threepences not minted at the Melbourne Mint exclusively, were minted in addition to the Melbourne coins at the following Mints - 1942 - San Francisco Mint "S", Denver Mint "D" 1943 - San Francisco Mint "S", Denver Mint "D" 1944 - San Francisco Mint "S" 1951 - London Mint "PL"
Then your coin was minted at the Philadelphia mint. It isn't an error, most older coins show no mintmark if the coin was minted at the Philadelphia mint, pennies still don't show a mintmark for coins minted at the Philadelphia mint.
Any US coin without a mint mark was minted in Philadelphia, because for a while, it was the only mint.