Regans' #6 was created by mixologist/bartender/author/cocktail historian/bon vivant Gary Regan sometime in the late 20th or early 21st century. Essentially, he couldn't find an orange bitters on the market that he liked, so he set out to make one. 6 recipes later, the formula was finalized and it went into production by the Sazerac Company at the Buffalo Trace Distillery. It looks old, but it's young. And delicious.
The flavour is of bitter orange peel and spice, specifically cardamom. Great in all manner of cocktails for those who prefer a spicier orange rather than a juicier, sweeter orange flavour like the Fee Brothers version.