The quality amazes me too. I taught my son to map read using a prismatic - in degrees of course - but in the school Cadet Force, they used silver compasses so he had to do a bit of quick re-learning. But the principle was the same. Out on the pretty desolate Cumberland fells he was pretty good and had to correct me a couple of times when it came to plotting our exact location on the map using back bearings. Hopefully some of it and the ability to live off the land for a couple of days will stick in his memory. He never quite grasped the magnetic variation thing though!
Just the work involved in engraving the old original case through 360 degrees with all the sector markings. Jeeeees, just how time consuming on an old pantograph engraver with the case set in an indexed rotating head.................