The Australian 1942 Threepence was minted at one of three mints.
The 800,000 coins minted at the Melbourne Mint have no mintmark.
The 8 million coins minted at the San Francisco Mint have a small "S" on the reverse below the units digit of the year.
The 16 million coins minted at the Denver Mint have a small "D" on the reverse below the units digit of the year.
The same applies for all Australian 1943 Threepences.
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.
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"
The rarest general circulation Australian Threepence, as distinct from the most valuable, would be the 1942 Threepence minted in Melbourne of which there were 520,000 minted. The other Threepences minted in 1942 were minted in the San Francisco (mintmark = S) and Denver (mintmark = D) mints in the USA. Threepences from this period that were struck in Melbourne have no mint mark. All others will have a mint mark on the reverse below the fourth digit of the year indicating the city where minting occured.
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.
Such a coin does not exist. All Australian Threepences from 1910 to 1944 inclusive were made from sterling silver.
KG is not a mintmark, it is the initials of the designer of the (kangaroo) reverse of the Australian 1938 to 1964 Penny and Halfpenny coins, George Kruger Gray. 1940 and 1941 Pennies minted at the Perth mint have a dot between the K and the G (K.G) as the mintmark.
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".
All Australian Pennies minted from 1938 to 1964 inclusive feature a kangaroo on the reverse. The only Australian coins to feature wheat was the 1938 to 1964 inclusive, silver Threepence coins.
All Australian Pennies minted from 1938 to 1964 inclusive feature a kangaroo on the reverse. The only Australian Coins to feature wheat was the 1938 to 1964 inclusive, silver Threepence coins.
From its initial issue in 1910, until 1915 inclusive, the Australian Shilling was minted at the Royal Mint London and has no mintmark. Some of the 1915 issue of the Australian Shilling were minted at the Heaton Mint in Birmingham and have a mintmark "H". From 1916 to 1926 inclusive, the Australian Shilling 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 1963, the Australian Shilling 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 1946 inclusive, Australian Shillings not minted at the Melbourne Mint exclusively, were minted in addition to the Melbourne coins at the following Mints - 1942 - San Francisco Mint "S" 1943 - San Francisco Mint "S" 1944 - San Francisco Mint "S" 1946 - Perth Mint - small dot (.) before the "S" in "SHILLING" (.SHILLING)
You do not specify a denomination or a year. Australian predecimal coins minted at the Royal Mint London prior to 1950, have no mintmark. Australian Halfpennies and Pennies minted at the Calcutta Mint in India in 1916 to 1918 inclusive, have an "I" mintmark below the bottom scroll on the reverse of the coins. Australian Halfpennies and Pennies minted at the Bombay Mint in India in 1942 and 1943, have an "I" mintmark below the head of George VI on the obverse of the coins.
Yes, the 1939 coin is worth more, but only in Mint Uncirculated condition, otherwise they are worth about the same. The 1942 "One Dot" Threepence is a minting error and represents about 8% of the 1942 Threepences minted.