It is extremely unlikely, but not entirely impossible that a 20 cent coin might have a 50 cent imprint on it however, the error would be very obvious since the 50 cent is 3 mm larger than the 20 cent coin.
Any coin with a "genuine" minting flaw would have some value, above the usual, as a collectible coin.
Genuinely flawed coins are not necessarily known about or documented until somebody turns up with one, since they are an "accident" of the minting process, and have escaped detection during quality control at the mint therefore, a valuation cannot be anticipated.
A reputable coin dealer should be able to identify and confirm the coin as genuine and make a valuation.
Decimal currency was not introduced into Australia until 1966. There is no such thing as an Australian pre 1966 cent.
The Australian 2 cent coin was introduced at the changeover to decimal currency in 1966. There were no Australian "cent" coins minted prior to 1966.
There is no Australian "cent" from 1939. Decimal currency was introduced in Australia in 1966.
There is no Australian "cent" from 1938. Decimal currency was introduced in Australia in 1966.
Such a coin does not exist. The Australian "cent" coins were first introduced in 1966.
There is no Australian 1954 20 cent coin. Decimal currency was introduced in 1966.
There is no Australian 1950 20 cent coin. Decimal currency was introduced in 1966.
Such a coin does not exist. The first Australian 20 cent coins were issued in 1966.
Such a coin does not exist. The first Australian 20 cent coins were issued in 1966.
Such a coin does not exist. The first Australian 10 cent coin was issued in 1966.
Such a coin does not exist. The Australian 2 cent coin was first issued in February 1966.
Such a coin does not exist. The Australian 2 cent coin was first issued in 1966 on the introduction of decimal currency.