Such a coin does not exist.
All Australian Threepences from 1910 to 1944 inclusive were made from sterling silver.
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.
Any collector value would depend on the country of origin and the condition of the coin.
There is no documented evidence of a 1942 Threepence with a blank reverse, but any genuine minting defect or irregularity would increase the value of the coin. A British 1942 Threepence (was only minted for Colonial use), uncirculated and in absolute mint condition could fetch up to £40 GBP. If it has been circulated but still in good condition, it might fetch up to £5 to £15 GBP. A British 1942 Threepence (dodecagonal - nickel brass), uncirculated and in absolute mint condition could fetch up to £7 GBP. If it has been circulated but still in good condition, it might fetch up to £2 GBP. A reputable coin dealer will be able to give a more accurate valuation.
There were no Australian coin year sets issued in 1942.
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.
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.
The small 50% silver Threepences (16mm) were still being minted for the Colonies from 1942 to 1945 inclusive. The changeover to the 12 sided nickel-brass Threepence (21mm) commenced gradually from 1937.
The Australian Threepence was minted in most years from 1910 to 1964 inclusive. Assuming they are in mint uncirculated condition, the most valuable coins not including pattern, proof or error coins would be - 1915 - $4,500 AUD 1942 - $3,700 AUD 1923 - $3,350 AUD 1914 - $2,650 AUD 1912 - $2,450 AUD The collector value descends rapidly as the condition of the coin decreases.
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.
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"
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.
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.