i saw something similar on the back of iceman's scope. They look cool.. and abit nicer then sticking a printed out protractor on the base. Are they expensive?
I bought mine from Mitre 10 for $10. It's useful for seeing what angle you're pointing at, especially if you go down the path of manual setting circles.
We used to have one on the sheet metal folder at work to show you the angle of the fold. Invaluable piece of equipment. Loupy, your terminology is correct.