Double bar 50 cent pieces 1979-1980
Although the original design for the 50c piece had two parallel bars behind the emu's head, the coins which were eventually produced for circulation did not. Reportedly the double bars reduced the life of the dies and were removed from the final design (http://www.cruzis-coins.com/50c/1980d.html). According to Renniks the double bars were removed from working dies after die filling was noticed around the double bars (p80, Skinner, Renniks Australian Coin and Banknote Values, 1989). Whatever the case, the double bars were not a part of the final production design. The 50c pieces of 1966 can show weak double bars, and faint double bars have appeared behind the emu's head in many years since 1966. Their occasional reappearance is probably due to incomplete removal from master tools, though it is possible that the faint double bars are nothing more than die deterioration, which the area behind the emu's head was found to be prone to.
In 1979 and 1980, a number of 50c pieces did have strong double bars behind the emu's head, much like those on a pattern 1966 50c piece (Crellin, https://www.sterlingcurrency.com.au/blog/news-research/decimal-coins-banknotes/the-pattern-australian-silver-50-by-machin/). The strong double bars on circulation coins are only known in 1979 and 1980 and while the reason is unknown, it is likely some dies were produced using master tooling where the double bars had not been removed at all. Although the double bar coins are the less common variety they are not rare. Oddly the usage of the double bar design increased in 1980 before disappearing altogether.
The double bars also appear on a small number of 1979 proof 50c pieces. It is unclear if they also appear on any 1980 proof 50c pieces.
Since 1980, proper double bars have only appeared on some C mintmarked dodecagonal 50c pieces in the 2015 50.50 two coin set for the 50th Anniversary of the Royal Australian Mint. Many of the Royal Australian Mint's press releases also show images of 50c pieces with strong double bars and no SD under the coat of arms.