We have all seen them, but what are they?
What are the Hawaii canx on US wine stamps? This is a nice example from Eric Jackson's website, and you can go buy it for your collection right now:

I did try to search for info on them, and I swear Ron Lesher may have done an article, but in my 31 seconds of trying to find anything I was unsuccessful. As such, I will take the easy way out and simply use your brains...
What are they and what is the story??
I have examples of at least the 2, 8, 20, and 40 cent denoms with the canx.