Hi Trev, The inscription sais 1 nonienteil = 1/16 dgr. So 1 stripe on the small scale is 1/16 degrees. I have found a similar one that was from 1 881 gun. They were placed on the breech area of the gun. Only difference with the other ones i found is that they are from 0 to 45 degrees, mine goes from 90 to 45 degrees. Is it possible that mine is from a mortar which is used a higher angle? See also here
http://www.lovettartillery.com/Foot Artillery Gunner's Quadrant.html